---start---- parasit 10/30 dr hunter immunity to protozoa no hooks, no rounds, no whips. basic immune mechanisms that deal with extracellular and intracellular parasites - understand them. have a knowledge of the basic mechanisms used by parasites to evade immune response have an understanding of how parasite and host factors can affect outcome of infection. immunity to protozoan infections: key features features of protozoan life cycles immune mechanisms that control infxn - humoral or CMI parasite and host factors that determine outcome of dz. life cycle features: small parasites. they're small. not large, like helminths. these are microparasites. often intracellular. they have complex lifecycles. host has to deal with many stages of parasite can cause chronic infections - common to many protozoan infections eg toxoplasma, trypanosomes, babesia, leishmania - once infected, you're infected for life. no sterile immunity. can't clear infection. often you have asymptomatic infection. rapid multiplication - contrast with helminths - one worm won't give rise to a lot of worms within you. but a single toxoplasma organism will produce thousands. these parasites also have sexual stages,can undergo genetic recombination, so have more genetic diversity - particular strains or isolates more virulent. outcome of host/parasite interaction can go two ways: parasite can be unsuccessful -in wrong host - not going to develop fully - eg avian schistosomes in human die in the skin. toxoplasma doesn't have this, but other protozoans do. babesia canis is dog only. -may encounter innate immunity - phagocytes, NK cells, dermis, complement - this is relevant to all kinds of infections. ability to recognize invading microbe and to kill it is important part of interaction. this can be at any level - cow tail flicking fly away, dermis of skin if thick, parasite may not be able to get through, phagocytes may eat invading microbe, complement will notice non-host surfaces and be activated via alternative or classical pathway and create MAC on parasite membrane -may enter immune host - antibody and CMI kick in. toxo, babesia, etc - infected hosts immune to reinfection. parasite can be successful: -requires appropriate host -survives innate response- many parasites have resistance to components of innate immune response. -requires non-immune host outcome of infection once established, can have several responses -host can develop appropriate immune response -adaptive immune response (B cell/T cell) can clear/control (often control, not clear- chronic disease is common) the infection -asymptomatic infection is common. as immune response develops there may be fever, other signs of some infection, caused by immune response. -host may have inappropriate immune response -fails to clear/control -clinical disease: death of host, development of severe pathology, chronic disease. if immune system makes wrong decision, can lead to clinical disease. in handout is a drawing on p 2 talkinga bout events leading to development of immune response...we've gone through this in immunology. immune response can be divided into two sections - humoral and cell mediated immunity. humoral: B, TH2 cells; CMI: TH1 cells. these responses are generally divided into TH1 associated with cellular immunity and TH2 associated with B cell response/Ab production. also interaction of accessory cells with parasites drives immune response toward one of these main types. humoral immunity: important for control of extracellular parasites -opsonization of parasites -ab dependent cell mediated cytotoxity (parasites bind antibody which brings in cells) -blocking function passive transfer of Ab. opsonization: immune system recognizes specific antigens. you start with relatively nonspecific antibody and end up with mature highly specific Ab response. Ab binds to parasite, leading to classical pathway complement activation and insertion of attack complex into parasite membrane. can also lead to uptake of parasite - many phagocytes have Fc receptor for Fc portion of Ab, so they bind the Fc and they eat up the parasites. then, you get a phagolysosome, and hydrolytic enzymes can destroy the parasites. also you have toxic free radicals which can kill the parasite - respiratory burst, NO, H2O2. important thing about Ab is it activates complement and causes opsonization. antibody can also inhibit receptor mediated entry of parasite. parasites use specific receptors to enter particular cells. if an antibody binds the receptor binding site on the parasite, the parasite's receptor binding site will not be recognized by its receptor. so parasite won't be able to enter cell and will not undergo its multiplication process. this is seen in particular with plasmodium (malaria) and other parasites. cell mediated immunity: important for conrol of intracellular parasites and viruses. can see cytolysis of infected cells- some parasites have intracellular life stages. within the infected cells cellular machinery picks up proteins elaborated by the parasites - and these proteins are then incorporated and presented on the cell surface (with MHC). this is recognized by the TcR on the surface of a cytotoxic T cell (CTL), leading to the degranulation of the CTL and the killing of the infected cell. the dead cell is no longer suitable host for parasites. death of cell often brings about death of intracellular parasites. cytotoxic enzymes punch holes in the cell membranes. activation of mphages to kill intracellular parasites- IFN gamma is made by T cells. this cytokine is critical to resistance against many intracellular pathogens - salmonella, plague, etc. why? IFNg doesn't kill parasites, but it indirectly acts to activate mphages, which then make toxic oxygen free radicals with then kill the parasites. mphages are often good host cells for the parasites. in the end, if mphages are given IFNg, they can kill the parasite specifically. IFNg is really really very important, very key mediator to resistance against many intracellular pathogens - due to ability to activate macrophages. evasion of host immune response: the parasites often due eventually survive host immune response. how? mechanisms include: intracellular life cycles: many spp use this mechanism babesia in rbcs, plasmodium in rbc, hepatocytes, t.gondii in any cell, leishmania in mphages, theileria in rbc, lymphocytes advantages of intracellular life: why is this attractive to parasite? well, if it's in a cell, it's protected from humoral immune response. a host making strong Antibody will still be infected if it is an intracellular parasite. also protected from complement and phagocytosis. you can evade classical and alternative pathways of complement activation. if you're in cell, you can't be phagocytosed, either. phagocytes won't recognize infected cells. this ability to invade and live in host cells has led to a codependence - parasites rely on host cells in some cases - parasties may need host biochemical machinery to stay alive. survival in macrophages - leishmania lives in mphages, these are the only cells it can live in. how does it survive in there? with toxoplasma, it inhibits the fusion of lysosome with the phagosome. T. cruzi instead allows the vacuole to fuse with lysosome, but it manages to escape from phagosome before lysosome fuses with phagosome. it lives in cytoplasm. leishmania is different. it lives in phagolysosome. we're not sure how. it's resistant to the low pH of 4 or 5 in there. so parasites can be resistant, can escape, or can be inhibitory. complex life cycle - toxo- tachyzoites, bradyzoites, oocysts, sporozoites - all antigenically different. can have switching from stage to stage as immune response switches. plasmodium too has many life stages within one host. if immune response is going to be effective it has to be against all the stages. infective stage is only in there for 2 minutes so it's hard to defend against. antigenic variation esp trypanosomes, see handout. african trypanosomes have variable surface glycoproteins which change frequently, protecting against complement mediated cytolysis (very thick) and protecting against the high levels of specifc antibody that develop. can make more than 1000 different genetically distinct coats. these are the paradigm for study of antigenic variation. similar aspects in babesia, plasmodium. [help! i'm getting sleepy! oh no, i have to stay up and study tonight!] anyway, there are different peaks in presence of particular antigen coat. misdirection of immune response- can lead to development of inappropraite response, can infect lymphocytes or mphages (immune cells), host genetics are important (dz is spectral) - eg some people get flu, some do not. mice infected with cutanous footpad lishmaniasis - some get TH1 resposne and resist infection, others get TH2 response, are susceptible, and eventaully die of dz. this is seen mainly in endemic areas. driven by genetics. anatomical site of infecton- parasites can live in different sites - away from immune response. babesia live in capillary beds, adhere to endothelium. spleen removes infected RBCs so babesia living in endothelium are safe from that. theileria, again, infects lymphocytes first - the very cell used to regulate the immune response. invasion of lymphocytes. brain of mouse. well developed cellular infiltrates. cysts of parasite within brain, simtulating zero inflammatory response. no immune response against cyst within brain. can be found in muscle. brain is immunocompromised site, remember, no T cells there. APC minimal. parasite factors that influence dz: initial infectious dose important with helminths with most protozoans, one parasite results in full infection. eimeria, cryptosporidium - self limitiing but high inectious dose can result in severe disase. host factors influencing dz: pathogenic fx of immune hyperactivity - fibrosis, granulomas immne complex formation and deposition overproducction of cytokines, making fever or toxich shock like reaction. some infections more likely to cause production of cytokines with pathogenic effects. splenomegaly. slide - heart muscle - asympotomatic t.cruzi infection in heart. extensive fibrosis has occured as a result of damage to patients. in part a consequence of overactiv e immune response. ag/ab complexes may also be released. polyclonal activation of immune response can lead to increased autoantibody titers which bind to self antigens and cause dz. host factors also include genetics (trypanosomes, leishmania), age, immune status - babesia will infect splenectomized humans. lymphomas, leukemias, BMT, AIDS - predispose people to toxoplasmosis. vaccine development is based on understanding how protective immnity is mediated. extracellular parasites - antibody response intracellular - CMI ---end---