Parasitology Today, voL 4, no. 4, 1988 89

Complement Mediated Evolution?

Apr i l

Complement - so called because it complements the immunologically-specific effects of antibody- is a vital component of the vertebrate immune system. The peptides of the Complement cascade activate macrophages, opsonize invading cells to facilitate their phagocytosis, and can also cause membrane lesions leading to direct lysis of invading cells. But amongst parasitic protozoa, not all stages are sensitive to Complement. Not all stages induce antibodies effective in initiating the Complement cascade, not all present appropriate markers for the Complement peptides, and some may even block biochemically.

Components of Complement are synthesized by macrophages and certain liver cells. These components are avail- able in the host bloodstream, ready to act against invading cells or bacteria. Thus, to survive in the bloodstream, parasitic protozoa must either be very quick, or be resistant to Complement, or they will succumb. Malaria sporozoites are very quick, rapidly invacling - hiding - i n host hepatocytes. Infective meta- cyclic trypanosomes, such as those of Trypanosoma cruzi, are not so quick. They can resist mammalian Comple- ment and so survive to infect mammals, but in birds they do not survive - killed by Complement-mediated lysis.

By contrast, those parasite stages in a blood-sucking insect vector do not encounter Complement, except for that remaining in the bloodmeal. The gut enzymes of blood-sucking ir sects such as mosquitoes can denature the Comple- ment peptides, and the parasite can then safely develop into Complement-sensi- tive stages. Clearly, if Complement is not present, there is no reason to maintain resistance to it.

As the 'strong right arm' of the verte- brate immune defence system, which is absent in invertebrates, Complement clearly represents a potential selection pressure in parasite evolution. Such effects may be quite subtle. In malaria for example, there are some species that infect birds, and others that infect humans. But, as Richard Carter and col- leagues have shown, chicken Comple- ment resists for 2-3 hours the action of mosquito midgut enzymes, while human Complement is very sensitive to them. So is this the site and mode of selection for the four human malarias? Malaria zygotes are specifically protected against the Complement systems of their host

ref lect ions on parasit ic p ro tozoa

species, but not generally against those of other species. Yet the parasites lose this protection as they develop into ookinetes and migrate into the mosquito midgut epithelium. Ookinetes of avian malarias tend to develop and migrate quickly, escaping, perhaps, the action of avian Complement remaining in the blood meal. In contrast, the ookinetes of human malarias such as Plasmodium fal- ciparum migrate more slowly and so survive only in a blood meal in which residual Complement has already been quickly denatured.

Trypanosoma cruz/in Opossums

No doubt the story is much more complex, and future issues of Parasito- logy Today will feature more detailed dis- cussion of these events. But the possible role of Complement in the evolution of parasitic protozoa may also bear strongly on the strange history of Trypanosoma cruzi. This parasite, that easily infects most mammals, is typically transmitted in the faeces of blood-suck- ing triatomine bugs. Bloodstream trypomastigote forms of the parasite,. which are Complement-resistant, are taken up by the bugs when they feed. In the bug gut, the parasites transform into Complement-sensitive epimastigote forms, multiply, and are then shed as Complement-resistant metacyclics that can infect a susceptible mammal. But the finding, by Maria Deane and colleagues in Brazil, that T. cruzi can also undergo this 'invertebrate' part of the development cycle within the anal glands of opossums, sheds new light on the possible evolution of this parasite.

Triatomine bugs appear to represent a relatively recent evolutionary develop- ment from predatory reduviid bugs, as- sociated with, but clearly subsequent to, the evolution of nest-building birds and mammals- such as the opposum. In con- trast, opossums are one of the oldest mammals in evolutionary terms - prob- ably arising over 70 million years ago. It is tempting therefore, to suggest that the association of T. cruzi with opossums may have preceded their association with triatomine bugs.

Opossums are omnivorous. They can feed on meat or viscera of infected ani- mals and may acquire T. cruzi infections in this way. They also feed on insects and plants that may contain flagellate proto-

zoans such as Crithidia, Leptomonas, Her- petomonas and Phytomonas. In general, such flagellates are Complement-sensi- tive, because, after all, they do not gener- ally encounter Complement and are not generally parasites of mammals. How- ever, the finding of epimastigotes (known to be Complement-sensitive) thriving in anal glands of opossums, shows that this must be a site where Complement is either absent or cannot act. Indeed, as Maria Deane and col- leagues point out, the anal glands of opossums can also harbour thriving populations of other flagellate species - particularly insect flagellates such as Crithidia.

There is some evidence that infective metacyclics of T. cruzi can be transmitted in the anal gland secretions of opossums (the parasites survive well in the urine of opossums, dogs and humans) so it may be that opossums represent not only an original reservoir, but also an original vector of the parasite. But the infect- ive metacyclics, being Complement- resistant, can enter the mammalian bloodstream - opening for themselves a new environment and the possibility of more widespread and more efficient transmission via a blood-sucking insect vector. Nor does that vector need to be a triatomine bug. T. cruzi can be transmit- ted (at least experimentally) by a variety of invertebrates, includingticks, bedbugs, leeches, and even houseflies and cock- roaches (the parasite will even thrive in the gut of some caterpillars). Triatomine bugs are now the main vector because they commonly inhabit the 'nests' of the mammalian reservoirs, and take large bloodmeals with a consequently high likelihood of encountering the sparse bloodstream parasites. They thus repre- sent 'good' vectors for the parasite to have associated with, and to have made further adaptations to.

So does T. cruzi merely represent an insect flagellate that has developed a Complement-resistant stage allowing it to pass from the anal glands to the bloodstream of opossums? Antigenically, the parasite is closely related to insect flagellates such as C. t-asciculata, L. col- losoma and l-lerpetomonas samuelpessoai which will confer partial protection to mice against challenge with 7-. cruzi. In contrast, there seems to be much less antigenic similarity between T. cruzi and other trypanosomes such as T. mega and T. melophagium, which confer much

~)i 988, Elsevier PubIJcatlons, Cambridge 016c'~1758/88/$02.00

90 Parasitology Today, voL 4, no. 4, 1988

lower levels of protection to mice chal- lenged with T. cruzi.

And how close is the relationship between T. cruzi and Leishmania? Both can block mammalian Complement, although while T. cruzi blocks it quickly, Leishmania blocks it later and accumu- lates C3b on its surface to help it bind to and invade macrophages. Some sandflies feed on plants, so it seems conceivable they may have acquired Complement-

sensitive plant flagellates which, on developing a mechanism that resists and manipulates Complement, could survive in mammals and become what we now know as Leishmania. Again, Comple- ment may have been a key selection fac- tor in the evolution of these parasites.

There is much more to be told, and much more to be discovered -especially as evidence for DNA and RNA sequence relationships between para-

sites becomes clearer. Yet April seems the month for such reflections which, hopefully, contribute not to confusion but merely to suggest a possible underly- ing theme. Many colleagues have contri- buted to these thoughts, but I will not name them because I alone must take responsibility for this precarious interpretation.

CJ. Schofield

Novel Source of Parasitological Information

C.A.M. Bell Seldom if ever have parasitological insights been found in the great works of literature. Important observations on the transmammary transmission of hookworm, however, may be found in a literary manuscript recently discovered in a Parisian archive. The concept of transmammary transmission is now well established, and while canine hookworm species are generally considered incapable of reaching maturity in man, they may well be capable of producing transient patent infections in neonates.

The manuscript, to judge by its literary style, is clearly a fragment written by Marcel Proust (1871-1922) for his great seven-part masterpiece A la Recherche du Temps Perdu (Remembrance of Things Past) and perhaps it will appear in future editions. The manuscript was discovered by Professor C.A.M. Bell, who has kindly provided the following translation.

My grandfather used to say, or rather not say in the sense of speaking aloud because his remarks might (his hearers being likely to repeat it in the manner of gossips everywhere) get back to the object of my grandfather's derision, but instead mutter in a manner that I under- stood perfectly well but which could not be captured felicitously by uncharitable ears or reproduced intelligibly by idle tongues: "He'll never do it. Swann's much too lazy to do the experiment. Oh, he'll swear it's true but he won't prove it out".

Fig. I. Swann" s hookworm experiment?

In this my grandfather, as was occasion- ally the case, was wrong. Swann's theory was bold, but just as a joyful caprice will lighten the spirit of him who indulges it, or a mournful dirge will weigh down the heart of the one who keens it, so Swann was emboldened by his idea and pro- ceeded resolutely. He had become con- vinced, the basis of that conviction being something that need not be considered here, for that would involve an arrange- ment of time and a perception of tem- poral events that he alone, and perhaps not even he, could capture in mere words (his eloquence in epistemological disquisition hardly extending to matters helminthological) that hookworms - those opalescent and surreptitious ser- pents in miniature which he had seen in the laboratory of M. d'Ancyloste, and about which he had become inordinately curious - are not necessarily transmitted by penetration of their miniscule pro- geny through the skin of unsuspecting persons. Surely, he thought, the larvae of those stalwart worms, being as peripate- tic in the organs of the body as he himself in the arrondissements of the City, might swarm in the swelling breasts of a new- delivered mother and thence be trans- ported to her suckling babe. This con-

ception, while gratifying to Swann qua philosopher, was insufficiently sophisti- cated for Swarm qua experimentalist, and he proceeded therefore to test a more challenging variant; that is to say he proceeded to test the proposition that the larvae of the dog variety of hook- worm could be transmitted to human beings through the milky conduits of a lactating bitch.

Obtaining a Roman dog of the she- wolf type - of which the Eternal City is justifiably if somewhat eccentrically proud - and making sure that it had recently whelped and was infected with hookworm, Swann embarked at once on his voyage of parasitological discovery, pausing only to marvel that it was he, if indeed it was he and not the mere remembrance of a certain practitioner of the art of research, who, in that time and place, if indeed it was that time and place and not some recollected atelier of another day, should be conducting such an exercise in natural history. He was lucky, as he had so often been lucky, in having as a friend Mme du Bini, who procured for him at no expense and with very little delay, two infant boys who had no mother, or rather no mother to speak of, and who were, in the minds of both Mme du Bini and himself, ideal subjects for the mission at hand. By the simple expedient of fostering the children on the gentle lac- tiferous beast (Fig. I) and, when the time was ripe, scrutinizing the product of the little ones' bowels with much care and a great deal of magnification, Swann proved (or rather demonstrated to his own artistic satisfaction)that hookworm larvae may, at least on occasion, quit one host and gain another by riding the snow-white lactary tide. Even my grandfather said he was right.