HIV Pathogenesis Update 2010: Immune Activation & Inflammation Richard Jefferys Coordinator, Michael Palm Basic Science, Vaccines & Prevention Project,

HIV Pathogenesis Update 2010: Immune Activation &

Inflammation

Richard JefferysCoordinator, Michael Palm Basic Science, Vaccines &

Prevention Project, Treatment Action [email protected]

XVIII International AIDS Conference, July 19, 2010

XVIII International AIDS Conference, 7/19/2010

What is Immune Activation?• Immunologists have different definitions, but

everyone has experienced the physical symptoms during acute infections like the flu, measles, mono: fever, swollen lymph nodes, fatigue

• Normally short-lived: these signs of immune activation happen, but only for a week or so

• Infection is then cleared or controlled, and activation subsides back to baseline

• After acute HIV infection, activation subsides, but not back to baseline


T-Cell Development

• T cells are produced in the bone marrow then travel to an organ called the thymus that’s just behind the breastbone.

• In the thymus, the T-cell acquires a “CD” surface marker that governs what type of T-cell it will be.

• The two major T-cell types are:– CD4 helper T-cell.– CD8 T-cells, including cytotoxic T-lymphocytes (CTLs) or killer T-

cells.


T-Cell Development• Both CD4 and CD8 T-cells have a docking bay type

structure called a "T-cell receptor" (TCR) that can dock with protein fragments called epitopes (from pathogens or other sources)

• TCRs are generated in the thymus in a sort of slot machine process that gives each T-cell one out of 25 million or so possible TCRs.

• A newly made T-cell leaves the thymus to patrol around the body looking for an epitope that fits its TCR. At this stage the T-cell is called “naïve.”


The Immune Response to Infection• On first exposure to a virus, incoming particles

are taken up by the sentries of the immune system, dendritic cells (DC)

• DCs can recognize pathogen-associated molecular patterns (PAMPs) shared by many different types of pathogens via toll-like receptors (TLRs)

• DCs become activated (switched on) which causes them to migrate from the site of exposure to lymph nodes


The Immune Response to Infection• DCs break the pathogen down into protein fragments

(called epitopes) which are then displayed on the outer surface by specialized molecules

• Class II HLA (also known as MHC) molecules present epitopes to CD4 T cells

• Class I HLA molecules present epitopes to CD8 T cells• In both cases recognition occurs via the docking bay

structure on the outside of the cell, the T cell receptor (TCR)



The Immune Response to Infection

• T cells travel through lymph nodes on string-like pathways made of fibroblastic reticular cells (FRC), these pathways form a complex traffic system with crossroads, junctions and dead ends

• DCs hang out at crossroads like salesmen trying to interest T cells in the epitopes they have on offer


Getting Activated• A passing naïve T cell that recognizes an epitope will

engage in a prolonged embrace with the DC and eventually become activated

Celli et al. Immunity. 27:625-634XVIII International AIDS Conference, 7/19/2010

Video of DC (green) and T cell (red) interactions in a mouse lymph node, before and after injection of an antigen. Note how the red T cells only contact DC briefly until antigen is present, then prolonged contacts can be seen.

Getting Activated• Activated T cells divide >15 times, generating a

swarm of T cells specific for the same pathogen epitope

• Dividing T cells switch on genes for making important signaling and antiviral proteins (chemokines & cytokines)

• Inflammatory cyokines and rapid T cell expansion contribute to the symptoms during acute infection (fever, malaise, swollen lymph nodes)


T Cell Subsets• Different T cell subsets engage in different tasks,

typically defined by production of particular cytokines

• CD4 T cells– Type 1 (Th1): help CD8 T cells kill infected cells– Type 2 (Th2): support production of antibodies by B

cells– Regulatory (Treg): release immune-suppressive

cytokines to dampen the immune response– Th17: Recently discovered subset involved in

responses to extracellular bacteria and autoimmunity


T Cell Subsets• CD8 T cells

– Recognize infected cells displaying pathogen fragments on their surface

– Release cell-killing substances (perforin, granzyme B) that puncture the cell wall and destroy the infected cell


The CD8 T cell is the smaller cell at the bottom of the image that punctures a larger influenza virus-infected cell and destroys it.

B Cells• Reside in lymph nodes and bone marrow • Factories for the production of antibodies

that can stick onto pathogens and prevent them infecting new cells

• Signals from CD4 T cells help them make antibodies that are more and more effective (“affinity maturation”)

• Recognize pathogen epitopes via B cell receptor (BCR)


Resolution & Memory• When the infection is controlled, most of the newly-

produced pathogen-specific “effector” T cells are no longer needed and die in a process called activation-induced cell death (AICD)

• Importantly, a subset of pathogen-specific T cell and B cells survive and these are described as “memory” cells

• Memory cells have enhanced functionality compared to naive cells and are often able prevent re-infection (with cleared pathogens) or control a pathogen that remains in the body (e.g. CMV, EBV, herpes zoster)

• End result is a team of memory CD4 T cells, CD8 T cells and B cells all targeting the same pathogen


Wherry & Ahmed, J. Virology, 78;11:5535-5545


T-Cell Pools


The thymus producesaround ~10-100 millionnew naïve T-cells every day (declines steadily with age)

Naïve T-cell pool size= ~100 billion

Memory T-cell pool size= ~200-300 billion

Naïve T-cells that hadn’t responded to anything die to make room for the fresh naïve T-cells

Naïve T cells thatmeet a matching antigen leave a legacy of memory cells which join the memory pool

Acute HIV infection

• Transient (typically) loss of CD4 T cells from blood, significant loss of CD4 T cells from gut

• High viral load• High levels of immune activation • Increased CD8 T cell counts & skewing of

CD4:CD8 ratio (normally around 2:1)


Acute HIV infection• HIV-specific immune responses become detectable in

2-3 weeks• Decline in viral load occurs in parallel with emergence

of HIV-specific memory CD8 T cell response but is rarely fully controlled

• HIV infects the CD4 T cells that are responding to it• Evidence of HIV-specific memory T cell dysfunction

emerges very early• Neutralizing antibodies are not generated for several

months and are rarely able to neutralize contemporaneous virus


HIV Infects Developing Memory CD4 T Cells

CD127 aka IL-7R is a marker for T cells destined to become long-term memory cells

Zaunders et al J. Virology, 80:20:10162-10172


Chronic HIV Infection• Immune activation does not fully resolve• Immune responses to HIV become

progressively more diverse– as the virus replicates, mutant forms arise and

these induce new immune responses (from the naïve T cell and B cell pools)

– effective immune responses pressure the virus to mutate in ways that prevent recognition, somewhat similar to the way HIV mutations can impair drug effectiveness (“immune escape”)


Immune Activation & Disease Progression

• Immune activation measured by a marker called CD38 on memory CD8 T cells is the strongest predictor of the speed of disease progression

• Significant link between immune activation and disease progression documented in every infected population, across all age groups and in every geographic location studied

• Immune activation correlated with progressive loss of naïve T cells and B cells (decreased response to new vaccinations)


Memory Cells Get Worn Out• Memory T cells become exhausted & senescent

– lose the capacity to proliferate (copy themselves)– sequential loss of cytokine production capacity: IL-

2>TNFalpha>interferon gamma– Express exhaustion markers (PD-1, Tim-3)– lose the CD28 co-stimulatory molecule, leading to an

accumulation of CD28- T cells (also seen in aging)– Shortened telomeres– Dysfunctional HIV-specific CD4 and CD8 T cells

accumulate

• Memory B cell exhaustion also documentedXVIII International AIDS Conference, 7/19/2010

T-Cell Pools Revisited


Decliningthymus production

Naïve T-cells decreased

Memory T-cell pool becomes less diverse, more dysfunctional

Persistent activation and recruitmentof naïve cellsinto the memory pool

Parallels with Aging• Decreased thymic output• Decreased naive CD4 and CD8 T cell numbers• Decreased response to vaccinations• Skewed CD4:CD8 ratio• Narrowing of the T cell repertoire, particularly in

CMV+ (memory pool gets crowded)• Increased numbers of CD28- CD8 T cells (associated

with morbidity & mortality)• Increased levels of inflammatory cytokines

(“inflammaging”)


Causes of Immune Activation

• Ongoing HIV replication: production of viral antigens and possibly also via viral HIV RNA stimulating toll-like receptors 7 & 8

• Microbial translocation - leaking of normally harmless bacteria from the gut into the circulation, leading to increased levels of bacterial DNA and LPS in the bloodstream


Causes of Immune Activation• Co-infections

– hepatitis C co-infection associated with increased CD8 T cell activation

– CMV: treatment with anti-CMV drug valganciclovir reduced CD8 T cell activation but did not increase CD4 T cell counts

– Other herpesviruses (Epstein-Barr Virus, Herpes Simplex Virus types 1+2)

– helminth infections• Loss of T cells leads to “homeostatic”

proliferation XVIII International AIDS Conference, 7/19/2010


Immune Activation Linked to Inflammation

• Ongoing activation of immune cells causes release of inflammation-promoting cytokines e.g. interleukin-6, tumor necrosis factor (TNF)-alpha, type 1 interferons

• Inflammatory damage to lymph nodes (fibrosis)• Additional biological markers of inflammation

such as C-reactive protein (CRP), fibrinogen and D-dimer can be elevated


Neuhaus J, et al. CROI 2009. Abstract O-140.

Participants 45-76 years of age

From SG Deeks, MD, at Atlanta, GA: March 2, 2010, IAS–USA.

Inflammatory Markers are Higher in HIV Infection Compared to Uninfected Controls

Impact of Virus Suppression• Immune activation declines rapidly• CD4 T cell increases: redistribution of cells

trapped in lymph tissue, proliferation of functional cells, production of new naive T cells from the thymus (slowest aspect of recovery)

• CD8 T cell numbers decrease• Memory T cell responses to opportunistic

pathogens improve• Memory T cell repertoire diversity improves



Antiretroviral Therapy Does Not Always Lower Immune Activation to Normal Levels

HIV Negative Untreated HAART0

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100

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Steve Deeks, IBT Workshop 2/20/2010


Inflammatory Markers Linked to Poor Health Outcomes

• IL-6, D-dimer & CRP associated with illness, frailty & mortality in the elderly (“inflammaging”)

• IL-6 & D-dimer levels strongly associated with mortality and non-fatal cardiovascular disease in the Strategies for the Management of AntiRetroviral Therapy (SMART) Trial (Kuller PLoS Med 2008)

• IL-6 & CRP strongly associated with opportunistic disease in SMART (Rodger J Infect Dis. 2009)

• Elevated levels of fibrinogen and CRP strong independent predictors or mortality in the FRAM study (922 HIV+ men & women from 16 US centers) (Tien CROI 2010)


Persistent Inflammation and Lack of Complete Immune Restoration May Contribute to Increased

Risk of Aging-Associated Diseases in HIV

• Cardiovascular disease• Cancer (non-AIDS)• Bone fractures/osteopenia• Liver disease• Kidney disease• Cognitive decline• Frailty

Multiple factors likely explain this increased risk, including co-morbid conditions and antiretroviral drug toxicity

From SG Deeks, MD, at Atlanta, GA: March 2, 2010, IAS–USA.

Research Issues

• Anti-inflammatory approaches (similar to those being studied in the elderly)

• Addressing other potential contributors e.g. stress, smoking, diet, lack of exercise

• Therapies to promote immune restoration• Safer antiretrovirals• Curing HIV infection


Documents

HIV Pathogenesis Update 2010: Immune Activation & Inflammation Richard Jefferys Coordinator, Michael Palm Basic Science, Vaccines & Prevention Project,