TrendsThe division between Leishmania para-sites causing tegumentary leishmania-sis (TL) versusvisceral leishmaniasis (VL)symptoms is less distinct than pre-viously thought, with increasing evi-dence suggesting overlappingsymptomatology as well as the spreadof TL-associated parasites to diversetissues and sites distant from the lesion.

Congenital transmission of VL-asso-ciated Leishmania species has beendescribed in humans; a transplacentalroute is believed to be responsible.Congenital transmission of TL-asso-ciated parasites has been documen-ted in animal models. Asymptomaticinfection with these parasites mayrepresent an unaccounted-for risk tomaternal–fetal health.

The maternal T cell response to Leish-mania infection has been shown to beassociatedwith adverse fetal outcomesin animal models. Emerging evidence isuncovering the relative rolesofparticularT cell subpopulations, cytokines, anddownstream effector cells in balancingcontrol of maternal infection and thegestational environment.

1University of Chicago Pritzker Schoolof Medicine, Chicago, IL, USA2University of Chicago ComerChildren’s Hospital, Section ofInfectious Disease, Chicago, IL, USA3Centro Internacional deEntrenamiento e InvestigacionesMédicas, Cali, Colombia4Instituto Nacional de PerinatologíaIsidro Espinosa de los Reyes, MexicoCity, Mexico

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ReviewPathophysiology ofLeishmania Infection duringPregnancyBrandon A. Berger,1,* Allison H. Bartlett,2

Nancy Gore Saravia,3 and Norma Galindo Sevilla4

The pathological processes resulting fromparasitic infection are known to haveimportant impacts on the mother child dyad during pregnancy. The roles ofparasite transmission and thematernal immune response have been describedin diseases such as malaria, toxoplasmosis, and trypanosomiasis. However,the impact of parasites of the genus Leishmania, etiological agents of theneglected tropical diseases tegumentary leishmaniasis (TL) and visceral leish-maniasis (VL), is comparatively less well known, though it is an increasinglyrecognized concern for infected mothers and their fetuses. In this review, wefirst consider the pathophysiology of placental infection and transplacentaltransmission of this parasite, and then discuss the role and mechanisms of thematernal immune system in simultaneously mediating maternal–fetal infectionand adverse pregnancy outcomes.

Leishmania and the LeishmaniasesThe leishmaniases are parasitic diseases caused by 20 species of kinetoplastid (see Glossary)protozoa of the genus Leishmania [1], which have traditionally been classified by clinicalsymptomatology. Visceral leishmaniasis (VL) is marked by fever and hepatosplenomegalywith immunosuppression and blood dyscrasias due to infection of the hematopoetic organs,and is [167_TD$DIFF]traditionally associated with infection by Leishmania infantum or Leishmania donovani.Tegumentary leishmaniasis (TL) is defined by nonhealing lesions of epithelial surfaces.Subtypes of TL include: cutaneous leishmaniasis (CL), which presents as nonhealing localizedulcers or nodules and is associated with over 15 geographically widespread species ofLeishmania; diffuse cutaneous leishmaniasis (DCL), which is characterized by multiple, oftenexuberant, lesions and is most associated with Leishmania amazonensis and Leishmaniamexicana in the Americas and Leishmania aethiopica in Africa; and mucocutaneous leishmani-asis (MCL), which is defined by involvement and destruction of mucosal tissues and cartilage,such as the palate or nasal septum, and is most frequently attributed to Leishmania braziliensisinfection. Current estimates of the incidence of the leishmaniases are 200 000–400 000 VLcases and 700 000 to 1.2 million TL cases per year, although underestimation and under-reporting of the disease is widely acknowledged [2,3].

Infected pregnant women raise unique clinical questions in terms of parasite distribution andhost immune response to the infection. The ability for infection of the mother to be transmittedto the fetus, or to affect the fetus as a result of the maternal immune response, is lessrecognized in Leishmania infection than in other intracellular protozoan diseases [4,5] suchas American trypanosomiasis [6], malaria [7,8], and toxoplasmosis [9]. Though the pathophysi-ological potential of Leishmania infection during pregnancy has been comparatively

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*Correspondence:bberger@uchicago.edu (B.A. Berger).

disregarded, recent evidence from infectious disease, obstetric, and immunological researchhas augmented the understanding of the anatomical distribution of the parasite, as well as thehost immune response as it relates to this special population. These recent advances areupsetting paradigms regarding the distribution of specific Leishmania species in the bodyduring infection, identifying high rates of transplacental transmission of the parasite in modelanimals, and defining the impact of the maternal immune response to infection on the health ofthe unborn child. This review aims to synthesize the state of knowledge regarding the infectiousand immunological impacts of Leishmania infection on the mother–child dyad and previewimportant frontiers of knowledge that may contribute to future understanding and clinicalmanagement.

The Pregnant Woman and the Clinical Spectrum of Leishmania InfectionPreviously, only VL was considered to be of risk during pregnancy as TL was presumed to beconfined to skin and mucosa. However, evolving understanding of th relationship betweeninfection by the various species of Leishmania and disease manifestations has brought intoquestion the traditional concept of VL as a systemic disease caused by a limited subset ofLeishmania species (L. donovani [168_TD$DIFF]and L. infantum) and TL as a localized disease caused bydiverse other species [1,10].

The manifestations attributable to a particular infection appear to be the result of interplaybetween factors such as dominant tissue tropism of the infecting Leishmania parasite and thehost immune and inflammatory response to their presence [11]. Within this framework, VL is theresult of predominant parasite presence in themononuclear [169_TD$DIFF]phagocyte system (MPS) cellsof the bonemarrow, spleen, and liver [12], while in TL parasites are predominately found inMPScells in skin or mucosal tissue. However, the predominant site of infection and clinicalsymptoms do not preclude dissemination of the infection to other anatomical sites [13–15].Furthermore, the state of relative immune tolerance induced by pregnancymay allow for greaterparasite dissemination and colonization of other organs and tissues. Clinical states such aspost-kala-azar dermal leishmaniasis [16,17] and visceral symptomatology attributable toTL-associated Leishmania species in animal models [18,19] and immunodeficient individuals[20–22] highlight the fluidity of the host–Leishmania relationship. Additionally, the recognition ofasymptomatic infection with Leishmania parasites [11,23], raises concern that women withoutclinical symptomatology may also be at risk for adverse pregnancy outcomes due to theundetected or unsuspected presence of parasites (Figure 1, Key Figure).

Furthermore, while unique immunological microenvironments appear to be located in areas ofconcentrated infection [24,25], the systemic distribution of cytokine products of the immuneresponse to localized Leishmania infection has been reported in conjunction with both VL[26,27] and TL [28]. As healthy gestation requires precise modulation of the maternal immu-nological milieu, Leishmania infections may affect fetal health by upsetting this balance at eitherthe local tissue or systemic level.

Placental Invasion and Congenital Transmission of the Leishmania ParasiteGiven the demonstration of parasite spread outside of the most commonly recognizedanatomical locations of infection, in both VL and TL [19,29,30], involvement of the placentais of clinical concern. Placental invasion may change the placental structure and lead to thepassage of the parasite to the fetus. Cases from the VL literature reporting congenitaltransmission (CT) in both symptomatic and asymptomatic women support the biologicalplausibility of this phenomenon [31,32]. However, the incidence of CT is likely to be greatlyunderestimated due to the lack of clinical recognition among healthcare providers and the

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GlossaryBlood dyscrasia: abnormality orimbalance of blood componentparts, such as red blood cells, whiteblood cells, and platelets.Complement system: a group ofimmune molecules that enhance, orcomplement, the ability of antibodiesand phagocytic cells to clearpathogens.Congenital transmission (CT):transmission of an infectious agent(bacteria, virus, or parasite) frommother to unborn child.Kinetoplastid: protozoan parasitesof the order Kinetoplastida, whichare defined by the presence of alarge mitochondrion with circularDNA. Examples includeTrypanosoma cruzi, which causesChagas’ disease, Trypanosomabrucei, which causes Africansleeping sickness, and Leishmaniaspp.Mast cell: a type of white blood cellinvolved in innate immunity; thesecells carry and release granules ofhistamine, an inflammatory molecule.Mononuclear phagocyte system(MPS): a family of phagocytic whiteblood cells distributed throughout thetissues of the body. Cells in thissystem engulf, and are infected by,the Leishmania parasite. Formerlyreferred to as the reticuloendothelialsystem.T cell: a type of white blood cellpredominantly involved in adaptivecell-mediated immunity. T cellsdifferentiate into a variety of types,including helper T cells (e.g., Th1, Th2,Th17) and regulatory T cells (Treg).Tegumentary leishmaniasis (TL):disease caused by Leishmaniainfection predominantly of the skin ormucous membranes, causing ulcersor nodules. Subdivided intocutaneous leishmaniasis (CL), whichpresents as nonhealing localizedulcers or nodules; diffuse cutaneousleishmaniasis (DCL), characterized bymultiple, often exuberant, lesions;and mucocutaneous leishmaniasis(MCL), which is defined byinvolvement and destruction ofmucosal tissues and cartilage.Visceral leishmaniasis (VL):disease caused by Leishmaniapredominantly of the spleen, liver,and bone marrow, causing cyclicfevers, hepatosplenomegaly, andabnormalities in blood cellproduction.

marginalized status of the majority of the pregnant women and babies at risk of mother-to-childtransmission of infection.

Congenital TransmissionRecent advances in knowledge of CT of Leishmania comes from studies of induced orobserved transmission in animal models and veterinary reports (Table 1). Among studiesshowing CT in animals, both TL-associated species [19,33] and VL-associated species[33–38] are represented. Furthermore, these studies demonstrated CT in both immunologicallypermissive laboratory animals [19,38] and wild-derived animals [33–37]. One study demon-strated CT but did not search for parasites in placental tissue [33]; all others support atransplacental route of CT with evidence of the parasite in placental tissue. To date, thetransplacental route is the only route that has been described for CT of Leishmania, and thesestudies represent ample proof of the capacity of Leishmania for placental invasion and CT viathe transplacental route, based on demonstration of parasites in placental, fetal, and newborntissue.

While animal studies have advanced our knowledge of invasion of the placenta by Leishmania,only one case showing Leishmania infection of the placenta in humans, from a case of knownmaternal VL with fetal demise, has been reported [29]. It should be noted that no other studyreporting inspection of placental tissue for Leishmania parasites in humans exists to ourknowledge, representing an important area for further research.

The question of differences among Leishmania species with regard to placental invasionremains unanswered. Leishmania infection of placental tissues in animal models has beendescribed for both VL-associated species, L. donovani [29] and L. infantum [34–38], as well asone TL-associated species, L. mexicana [19]. The low number of studies intentionally searchingfor placental infection means that placental tropism by other species is by nomeans precluded.Strains within species may also differ in propensity for placental invasion.

When considering the applicability of these animal studies to human health, differences inplacentation and immune state between animal models and humans should be considered.Human andmurine placentation is hemochorial, meaning that maternal blood comes into directcontact with the fetal chorion, and therefore requires greater maternal immune tolerance. Incontrast, canine placentation is endotheliochorial, which is less invasive and requires lessermaternal immune tolerance [39]. The hemochorial placentation in mice and humans allows forgreater permeability, by both parasites and antibodies, than does endotheliochorial placenta-tion. It has been hypothesized that a placental structure allowing for considerable fetal exposureto infectious and immunological components in the maternal blood has been maintained fromancestral placental structure to provide increased passage of maternal immunological capacityto the fetus, thereby protecting the newborn from postnatal infections. This benefit is counter-balanced, however, by increased risk for prenatal infection of the fetus [40]. While no firmconclusions may be drawn at this point regarding the relative Leishmania infection risk of thesetwo placentation types, this difference in placental anatomy raises questions regarding the rateof transmission in infected humans versus that observed in canine studies, and has implicationsfor appropriate future animal modeling of CT of Leishmania.

Further limiting the generalizability to humans, the mice studied thus far have been of the BALB/c strain, a model strain that is more immunologically permissive to intracellular parasites thanwild-derived mice [41,42]. Future animal models of placental invasion and transplacental CTshould endeavor for representative placental anatomy and immune states to increase ability forextrapolation of results.

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Key Figure

Pathophysiology of Leishmania Infection in Pregnancy

Leishmaniainfec�on

Systemicparasite

distribu�on

Asymptoma�cinfec�on

Maternalimmuneresponse

Fetal effects

Containment

Robustresponse

Insufficient

to contain

infec�on

Leishmaniasis(VL/TL symptoms)

Alteredplacentalfunc�on

Congenitaltransmission

Infec�onleared

Intoleranceto

foreign/fetalan�gensExacerbated

maternal diseasePlacental invasion

Pregnancy

Permissiveimmuneresponse

ma�con

ntai

ngenitalsmission

Infec�onleared

Intoleranceto

foreign/fetalan�gens

Leishmaniasis(VL/TL symptom

E b t d

Permissiveimmuneresponse

1

2

3g

6

4

5

tedtalon

Insuffi

to conta

infec�onRobust

response

nment

e

Figure 1.

(Figure legend continued on the bottom of the next page.)

Leishmania infection during pregnancy presents multiple mechanisms for potential maternal–fetal pathophysiology. Following infection, the pregnantmaternal immune system responds to the parasitic invader. If the response is unable to control the infection, leishmaniasis will occur, with either visceral or tegumentarysymptoms (1). Increased maternal tolerance of foreign antigens during pregnancy may also lead to exacerbated symptomatic disease (2). Either of these cases mayultimately permit invasion and colonization of placental tissue (3), causing placental pathology and/or CT. If the maternal immune system controls the parasite without

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Table 1. Reports of Placental Infection and Congenital Transmission of Leishmania in Animal Models and Veterinary Reports

Animal Leishmaniaspecies

Birth PlacentasPCR+a

OffspringPCR+

Notes Refs

Mouse (BALB/c) L. mexicana Motherssacrificed beforeparturitionb

110/110 39/110(fetuses)

Higher number of fetal resorptions and deaths inthe infected groupBALB/c mice are susceptible and unable tomount a competent response to Leishmania

[19]

Hamster (Syrian golden) L. panamensisL. donovani

Vaginal delivery Not assessed 24/93(L.panamensis)11/75(L.donovani)

Lymphoproliferation more common inL. panamensisLower weight gain in offspring in L. donovanigroup

[33]

Canine(foxhound)

L. infantum Vaginal delivery Yesc 10/12 Three PCR+ pups with immunological/parasitological progression followed over 6years; reported in [92]

[34,92]

Canine(NSd)

L. infantum Vaginal delivery Yesc 2/2 Pups stillborn [35]

Canine(mixed breed)

L. infantume Necropsy orovario-hysterectomyb [151_TD$DIFF]

26/53 17/53(fetuses)

Offspring from four symptomatic mothers andfour asymptomatic mothersNo difference in transmission between thegroups

[36]

Canine(Beagle)

L. infantum Cesareansectionb [151_TD$DIFF]

2/3 3/3 Uterus PCR+Fourth pup was autolytic and not analyzedThree other intentionally infected mothersunable to gestate pregnanciesAll mothers bred to chronically infected male

[37]

Mouse (BALB/c) L. infantum Motherssacrificed beforeparturitionb [151_TD$DIFF]

3/16 4/88(fetuses)

1/8 male mice was PCR+ after mating withinfected femaleBALB/c mice are susceptible and unable tomount a competent response to Leishmania

[38]

aPCR+: polymerase chain reaction positive for the presence of [152_TD$DIFF]Leishmania DNA.bFor the purpose of avoiding confounding transmission in the birth canal.cPlacenta reported in singular.dNS: not specified.eLeishmania chagasi is now considered synonymous with Leishmania infantum [93].

Placental PathologyPlacental invasion by parasites also carries risks for adverse fetal outcomes apart from CT.Leishmania infection has been reported to cause villous vessel thrombosis and trophoblasticdegeneration of the placenta [29,43]. Such placental pathophysiology is associated withoutcomes such as severe maternal disease, preterm labor, intrauterine growth restriction,and fetal demise [43–46]. The occurrence of such outcomes as a result of Leishmania infectionis suggested by the large number of failed pregnancies reported by the animal model studies[19,35,37], as well as increased rates of fetal complications in human cases of concurrentpregnancy and Leishmania infection [32,47,48]. However, rates of these outcomes are pre-sented for comparison alongside a control group in only one study [19]. Carefully designedresearch with intentional measurement of these pregnancy outcomes is needed to understandthe full spectrum of effects of placental invasion by Leishmania and its epidemiology in both

fully eliminating it, rendering the mother asymptomatic, the fetus remains at risk (4) for pathological effects of the parasite. Conversely, a stronger maternal immuneresponse may clear the parasitic infection (5), but may also directly affect the fetus via cell-mediated immunity, cytokine milieu, and reduced maternal tolerance to non-self (6). In summation, the maternal immune response simultaneously determines the likelihood of the parasitic infection of the placenta and fetus, and the hospitality orhostility of the gestational immune environment. VL: visceral leishmaniasis; TL: tegumentary leishmaniasis; CT: congenital transmission.

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animals and humans. Ethical considerations will be crucial to designing research that canconcurrently detect, measure, and intervene on this pathology in human cases.

Given the tropism of Leishmania for the cells of the MPS [49,50], it is hypothesized thatHofbauer cells, the macrophages of the placenta, are the primary placental cell line targeted byLeishmania. Macrophage infection by other congenitally transmittedmicrobes, such as the Zikavirus [51,52] and Trypanosoma cruzi [53,54], has been suggested as a possible mechanism forplacental invasion and spread across the placental barrier to the fetus.

Another possible mechanism for transmission across the placenta, parasite-mediated break-down of the trophoblast, has been demonstrated to promote the spread of intracellularprotozoa such as T. cruzi and Toxoplasma gondii through the placenta to the fetus [55,56].Whether a similar mechanical weakening of the maternal–fetal barriers enables Leishmaniainfection to spread across the placenta requires further investigation.

The Role of Timing of InfectionTransmission risk may vary depending on when the maternal infection is acquired, as it doeswith other congenitally transmitted protozoan infections. In the case of T. gondii, only acuteinfection during pregnancy is considered a risk to the fetus, and fetal outcomes are worse wheninfection occurs in the first trimester compared with the second and third trimesters [57]. Bycomparison, T. cruzi transmission may also occur in cases of acute or chronic maternalinfections, including those that are asymptomatic [6,47]. Furthermore, the time-dependentcondition of the maternal immune response, whether acute, chronic, anergic, or exhausted[58], will impact parasite viability and the potential of the microbe to affect the fetus. Thesefactors remain unexplored in the case of CT of Leishmania.

Maternal Immunological Response: Balancing Parasite Invasion andGestationThe maternal immunological environment during concurrent pregnancy and Leishmania infec-tion is also a potential source of pathophysiology. The immunological response to normalpregnancy is an adjustment in which the maternal immune system adapts to permit non-selfantigens inherent to gestation while simultaneously maintaining defense of mother and childagainst infectious microorganisms. This broadly implies reduced proinflammatory immuneresponses with maintained humoral immune responses, especially at the level of the placenta,which has been shown to act as a regulator of this unique immune state [59].

As such, in the case of Leishmania infection during pregnancy, the maternal immune responseis challenged by the antagonistic demands of maintaining the mother’s increased tolerance offetal antigens and the necessity for response to an intracellular microbe, which is not effectivelycleared by humoral immunity [24]. The clinical outcome of mother and fetus is a reflection of theresulting compromise between healthy gestation of the pregnancy and effective response tothe invading parasite [57].

Healthy pregnancy has been associated with specific T cell balances, notably type 2 helper Tcells (Th2) predominance over type 1 helper T cells (Th1), and regulatory T cell (Treg) predomi-nance over type 17 helper T cells (Th17). While certain exceptions to this anti-inflammatorytrend do exist in normal pregnancy [59], pregnancy loss, premature labor, and other compli-cations of pregnancy are associated with deviations from these cytokine trends [60,61].

To date, the most studied aspect of this balance in concurrent Leishmania infection andpregnancy has been Th1/Th2 differentiation and resultant cytokine patterns. Th1 and Th2promote distinct cytokine environments with feedback mechanisms that are generally self-

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reinforcing and antagonistic to each other. [170_TD$DIFF]Research based on Leishmania infected pregnantC57BL/6 mice, [171_TD$DIFF]which are capable of both Th responses, [172_TD$DIFF]has established a rudimentarydichotomy regarding Th environments [173_TD$DIFF][62–65]. In this paradigm, a Th1-predominant inflam-matory immune response [174_TD$DIFF]is associated with both rejection of pregnancy and control ofintracellular Leishmania infections [66], while a Th2-predominant response [174_TD$DIFF]is associated withan immune state tolerant to foreign antigens, including allogeneic antigens of the fetus,manifesting as preservation of pregnancy and poor Leishmania infection control [67,68]. Thesefindings have helped to establish the groundwork of understanding [175_TD$DIFF]of T cell response to otherprotozoan infections [69], including during pregnancy [70].

However, as our understanding of T cell taxonomy and differentiation has further evolved,regulatory T cells (Treg) and Th17 cells have emerged as crucial immune markers andmodulators of both protozoan infection and pregnancy. Inflammatory Th17 activity is down-regulated during normal pregnancy, with concomitant increased Treg activity [60]. While similarto the previously described Th1/Th2 relationship in that Th17/Treg environments interact withand balance each other, the impacts of these cytokines do not follow the same pattern ofassociation as Th1/Th2 (Figure 2). That is, while Th17 is hostile to pregnancy, it is alsoassociated with severe disease manifestations in leishmaniasis [71,72], as well as other relatedintracellular protozoan infections such as T. cruzi [73] and Neosporum caninum [74]. Treg cellson the other hand have been shown to contribute to the control of Leishmania infection inanimal models [75], and are associated with maintenance of pregnancy. To date, Th17 andTreg profiles have not been measured in concurrent Leishmania infection and pregnancy.

It should be noted that T cell responses in vivo are not mutually exclusive, nor are their effectsbinary, as these are cytokine milieus determined by multifactorial feedback loops among thedistinct Th types [60]. The most likely result of the competing demands of pregnancy andLeishmania infection is an antagonistic cytokine milieu that is potentially more permissive toinfection due to themodulation of the immune response by pregnancy, primarily due to Th2 andTreg responses, as well as potentially detrimental to the fetus, primarily as a result of proin-flammatory Th1 and Th17 responses. A relative increase in expression of all T cell profilescompared to noninfected, nonpregnant organisms is expected, though each of theseresponses is downregulated somewhat by the other, leading to a complex semipermissiveimmune state for both fetus and parasite.

Furthermore, the effects of the various discussed T cell types should not be characterized asdichotomously anti- or progestational. For example, the Th2-predominant environmentinduced by, and beneficial for, the maintenance of pregnancy also appears to favor thepersistence of Leishmania infection in the mother, potentially increasing the risk of CT, whichis ultimately detrimental to the fetus. Conversely, an antiparasitic Th1-predominant responsedecreased CT in dogs [47], indicating a potential benefit to surviving offspring of an immuneresponse that could compromise gestation. T cell impacts on both infection and gestation aredependent on themultiple downstream cytokines produced, rather than on the T cell itself, so aparticular environment may promote both progestational and antigestational effects [76].Finally, T cell differentiation is associated with a range of factors independent of Leishmaniainfection, including genetic background [77], hormone production [66], age [78], nutritionalstatus, and comorbidities [79], which influence T cell environments and clinical outcomes.

Frontiers of Knowledge Regarding Immune System Modulators and ComponentsLittle has been published regarding the interplay of pregnancy and Leishmania infection withvarious important upstream and downstream mechanisms that affect immunological parasitecontrol. We therefore highlight several areas of interest for future research (Table 2) – namely,

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MaternalT cell

differen�a�on

Inflammatory(IFN-γ,TNF-α,

IL-17)

Regulatory(IL-4, IL-10,

TGF-β)

Related immunological processes of interest:-an�body produc�on-complement (via an�body-an�gen coupling)-cast cell homing

Related immunological processes of interest:-phagocytes (e.g. neutrophils)-complement (via IL-17)

Hos�leto pregnancy

Infec�oncontrolled

Possible:IUGR,

preterm birth,fetal loss,

SAB

Possible:extensivematernal

disease, CT

Permissiveto pregnancy

Infec�onuncontrolled

Healthymother and

gesta�on

Th2

Treg

Th1

Th17

l

Figure 2. PostulatedMaternal T Cell Responses and Downstream Immune Effects during Concurrent Pregnancy and Leishmania Infection.MaternalT cell differentiation impacts responses to both pregnancy and infection. Broadly, maternal immune conditions during pregnancy are typically predominantly regulatory(Th2/Treg). However, if the maternal response to Leishmania infection is predominantly inflammatory (associated with Th1/Th17 responses), it may have deleteriouseffects on gestation. Th17/Th2 predominant responses may be conducive to exacerbated disease in the mother and the potential for transmission to the fetus in thecase of Leishmania infection. It should be noted that in vivo immune environments are not exclusive to any T cell class, but are rather predominated by certain cytokineprofiles and effects. Th: Helper T cell; Treg: regulator T cell; IL: interleukin; IFN-g: interferon gamma; TNF-a: tumor necrosis factor alpha; TGF-b: tumor growth factorbeta; IUGR: intrauterine growth restriction; SAB: spontaneous abortion; CT: congenital transmission. Adapted from [67].

possible roles for sex hormones, phagocytes, mast cells, the complement system, andantibodies – in determining disease and pregnancy outcomes for the mother–fetus dyad.

Additionally, it bears stating that direct observational description of the human maternalimmune milieu in concurrent pregnancy and Leishmania infection remains rudimentary. Onlyone study of two women with TL has endeavored to quantify and compare serum immunologi-cal components in pregnant women infected by Leishmania to control groups of healthy andinfected women [80]. Further case reporting is required to establish associations with statisticalsignificance between infection, maternal immune response, and fetal effects.

Potential Impact on the Immune Development of the NewbornThe interplay of the maternal immune system with the developing fetal immune system is ofemerging importance in all maternal infections [81,82]. For example, transplacentally sharedmaternal antibodies may impact the development of the immune systems of uninfected

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Table 2. Potential Impacts of Select Immune Modulators and Effectors on the Pathogenesis of LeishmaniaInfection during Pregnancy

Component Role Potential effect Refs

Sex hormones Immune modulator Progesterone and estrogen levels are elevated duringpregnancy

[66]

[153_TD$DIFF]Estrogen has been shown to increase NOa[154_TD$DIFF] mediated

Leishmania killing in hamsters[94]

[155_TD$DIFF]Progesterone’s effect has not been described inLeishmania infection, but has been shown to promote aTh2b [156_TD$DIFF] milieu and suppress innate immune systemcomponents involved in parasite killing

[66,67]

[157_TD$DIFF]Testosterone appears to increase risk for parasiticinfections

[95–97]

Mast cells Immune modulator/effector

Granule contents such as histamine, serotonin, TNF-ac[158_TD$DIFF],

and prostaglandin F2a are associated with expulsion ofuterine contents

[98]

[159_TD$DIFF]Mast cells have been noted in Leishmania-infected tissuesin mouse models

[99,100]

Complement system Immune effector High levels of complement activation have been shown tolead to increased fetal loss

[101]

[160_TD$DIFF]Complement activation due to infection by Plasmodiumhas been identified as a mechanism of parasite-inducedimmunologic pathology of the placenta

[102]

Antibodies Immune effector The role of IgGd[161_TD$DIFF], the antibody class able to cross the

placenta, in fighting intracellular infections is an ongoingfield of study

[103]

[162_TD$DIFF]Anti-Leishmania antibodies are known to cross theplacental barrier in cases of maternal infection

[31]

a[163_TD$DIFF]NO: nitric oxide; bTh2: type 2 helper T cell; c

[164_TD$DIFF]TNF-a: tumor necrosis factor alpha[165_TD$DIFF]; dIgG: immunoglobulin G.

Outstanding QuestionsDo TL-associated species of Leish-mania invade and/or traverse the pla-centa in humans?

What is the mechanism by which theLeishmania parasite crosses the pla-cental barrier?

How does acute versus chronic mater-nal Leishmania infection affect [177_TD$DIFF]risk ofcongenital transmission and [178_TD$DIFF]maternalimmune responses [179_TD$DIFF]antagonistic topregnancy?

What roles do mast cells, antibodies,the complement system, and sex hor-mones play in determining maternal–fetal health in simultaneous Leish-mania infection and gestation? Howare these components related toknown patterns of T cell differentiationduring infection and pregnancy?

How can knowledge of the pathophys-iological mechanisms of [180_TD$DIFF]Leishmaniainfection during pregnancy inform clin-ical management of this population?

offspring, as some neonates have been observed to exhibit diminished immunologicalresponse to parasitic infection as infants following exposure to maternal antibodies to parasiticinfection in utero [83,84]. While data from malarial and helminthic parasitic infections duringgestation have been shown to increase offspring susceptibility to the same species withpostnatal challenge [84–86], data regarding postnatal disease susceptibility are unavailablefor Leishmania.

Concluding RemarksWhile the pathology of simultaneous infection and pregnancy is well recognized for a variety ofintracellular protozoa, understanding and recognition of the pathology caused by Leishmaniaduring pregnancy has lagged. Adverse pregnancy outcomes have been reported in the clinicalliterature associated with this parasite, and emerging understanding of the systemic impacts ofLeishmania – including spread to anatomical areas previously considered unaffected andinteraction with the host immune response – has heightened interest in the impact of leish-maniasis on the pregnant woman and her fetus. While human cases and animal models showthat placental invasion and CT do occur, and a theoretical framework for how the parasite maycross the placental barrier is presented, the mechanism remains unproven. Of immediateinterest is the question of whether TL-associated Leishmania parasites cross the placenta inhumans and therefore pose a CT risk, a question that will need to be answered throughintentional observational research in endemic areas (see Outstanding Questions).

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Additionally, a groundwork understanding of the opposing demands of the immune responsesto Leishmania infection and pregnancy has been laid, particularly with regard to the generalroles of differentiated T cells, as recent advances separately describing the immune parametersof the two processes have allowed for reasonable triangulation of possible pathophysiology ofleishmaniasis during pregnancy. However, the paucity of studies of immune response inhumans limits the utility and generalizability of current knowledge to human leishmaniasis.Future investigations are needed to clarify the roles of the various elements of both the innateand adaptive immune systems in humans, the impact of timing and chronicity of infection, andthe variability of responses elicited by different Leishmania species on pregnancy and preg-nancy outcomes. In the short term, comparative observation of immune parameters andpregnancy outcomes in women infected prior to and during pregnancy would be helpful inelucidating aspects of the immune response unique to the pregnant state, as well as the impactof the chronology of infection and pregnancy. Further illumination of factors such as parasiteload, parasite species, and maternal comorbidities, with a focus on human studies, is alsoneeded.

Finally, application of new knowledge to themanagement of affectedwomen and their offspringis of considerable priority. Current international guidelines provide broad advice for clinicians,though simultaneously cite the lack of evidence available for making decisions regardingoptimal management of pregnant women with clinical disease [3,87]. Evidence for the effectsof anti-Leishmania drugs in pregnancy is scare to nonexistent [88]. At present, VL in pregnantwomen is treated with amphotericin B, based on the risk to maternal and fetal life posed by thatillness and reports of successful treatment and few adverse events using that drug [89–91]. TLis currently not routinely systemically treated during pregnancy, as risk to the fetus is poorlydefined for both the disease [176_TD$DIFF]and available drugs; local pharmacologic, thermal, or cryotherapytreatment may be considered, based on assessment of risk versus benefit [3,87]. Continuedadvances in our understanding of the pathophysiology of the parasite in pregnant women standto more precisely define the risks posed by this parasite and improve care for mother and child.

AcknowledgmentsThis review was supported by the University of Chicago Pritzker School of Medicine Pritzker Fellowship (BAB) and the

American Society [181_TD$DIFF]of Tropical Medicine and Hygiene Benjamin H. Kean Travel Fellowship (BAB). We wish to thank the

librarians of the University of Chicago for their considerable help in obtaining articles.

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