The role of T-cell subsets and cytokines in the regulation of infection

Phillip Scott and Stefan H.E. Kaufmann

The presence and role of CD4 + T-cell subsets in disease

The division of CD4 + T ceils into distinct subsets (TH1 and TH2), based upon cytokine production, is a major advance in our understanding of immune responses, particularly those associated with infectious disease 1. Pathogens often appear to stimulate only one CD4 + subset (Table 1); for example, in the mouse many helminth parasites, such as Heligmosomoides polygyrus and Nippostrongylus brasiliensis, prefer- entially induce TH2 responses 2 (F. Finkelman, Bethesda), while bac- teria (e.g. Brucella abortus) stimu- late TH1 responses (T. Mosmann, Edmonton). Analogous responses oc- cur in human disease, as shown by S. Romagnani (Florence) who derived a large panel of human T-cell clones specific for bacterial (purified pro- tein derivative (PPD)) and helminth (Toxocara canis) antigens that could clearly be defined as THt or TH2 cells, respectively 3. Similarly, patients with filaria-induced eosino- philia exhibit a significantly greater frequency of IL-5-producing T cells than uninfected individuals (T. Nutman, Bethesda).

Other pathogens are associated with stimulation of different subsets at different stages of infection. In murine schistosomiasis (E. Pearce, Ithaca) and murine malaria (J. Langhorne, Freiburg), THt cells pre- dominate during the initial stages of infection, but are replaced by TH2 cells during the later stages of infec- tion. The loss of THl-type responses also occurs during murine retro- virus-induced immunodeficiency, with the development of enhanced TH2 responses (H. Morse, Beth- esda). Interestingly, G. Shearer (Bethesda) reported that a similar sequence may occur in humans dur- ing HIV infection. An alternative to

The study of immunity to infectious agents has proved valuable for analys- ing the role ofT-cell subsets. This was true when Zinkernagel and Doherty demonstrated MHC restriction using viral and bacterial systems, and was re- emphasized at a recent meeting ~'~, where data were presented on the role of T cells and cytokines in viral, bac- terial, protozoan and helminth infec- tions. This report summarizes some of

the highlights.

sequential activation is simultaneous TH1 and TH2 responsiveness at sep- arate sites; for example TH1 and TH2 responses are observed during murine Trichinella infection, but in different lymphoid organs, stressing that these responses can be compart- mentalized (D. Wassom, Madison). Compartmentalization is also seen in routine cytomegalovirus (CMV) infection: CD8 + T cells are essential for protection in all organs except the salivary glands, where CD4 + T cells control infection (U. Koszinowski, Ulm). In leishmaniasis, the nature of the response is depen- dent upon the mouse strain infected. TH1 cells develop after infection of resistant mice and mediate protec- tion, while TH2 cells are stimulated in the susceptible BALB/c mouse 4. Similarly, in leprosy patients, inter- leukin 4 (IL-4)-producing cells seem to occur in the lepromatous form, whereas gamma-interferon (IFN-y)- producing cells seem to dominate in the tuberculoid form (R. Modlin, Los Angeles). One of the most inter- esting findings here is that 'sup- pressor' CD8 + T-cell clones produce IL-4, suggesting that some CD8 + cells may have a TH2-1ike cytokine

*The symposium on The Role of T-cell Subsets and Cytokines in the Regulation of Infection was held at the Mary Lasker Center at the National Institutes of Health, Bethesda, Maryland, on 6-9 May, 1991 and was organized by R. Coffman, S.H.E. Kaufmann and A. Sher.

1991, Elsevier Science Publishers Ltd, UK. 0167 4919/91/502.00

profile (B. Bloom, New York). Although one can predict the

nature of the immune responses that are associated with stimulation of a particular T H subset, whether those responses will be protective depends upon the specific pathogen. F. Finkelman reported that immunity to reinfection with Heligmoso- moides polygyrus is IL-4 depen- dent, although it does not appear to be IgE dependent. In contrast, increased resistance to infection with other helminth parasites, such as Schistosoma mansoni (E. Pearce) and Trichinella spiralis (D. Wassom), appears to be associated with production of the TH1 cyto- kines IFN-y and IL-2. T H subsets also mediate immunopathologic responses. For example, G. Grau (Geneva) found that mouse cer- ebral malaria is associated with en- hanced production of IFN-y, and that treatment of mice with anti- IFN-y protected these animals against the development of cerebral malaria. Conversely, in murine schistosomiasis, immunopathologic responses correlate with the pres- ence Of TH2 cells (A. Sher, Bethesda). The confusion in this area may be eliminated as effector mechanisms are further defined. Increasing evi- dence suggests that reactive nitrogen metabolites, rather than reactive oxygen metabolites, represent the crucial effector molecules respon- sible for the anti-microbial functions of macrophages; this has been shown most clearly in murine leishmaniasis and tuberculosis (E. Liew, Beckenham; M. Meltzer, Rockville; S. Kaufmann, Ulm) s. Sev- eral cytokines, including IL-4, IL-10 and transforming growth factor /3 (TGF-[3) can interfere with IFN-y- mediated macrophage activation by inhibiting nitric oxide production (E. Liew; A. Sher; M. Meltzer).

Immunology Today 346 Vol 12 No. 10 1991

CD4 + T-cell subsets: differentiation and regulation

Several factors can influence the development of T H subsets, includ- ing the antigen, the antigen con- centration, the route of antigen administration and the antigen- presenting cells (P. Bretscher, Saska- toon; E. Liew; A. Lichtman, Boston; F. Fitch, Chicago) 6-8. K. Bottomly (New Haven) described one system where the response to a single pep- tide epitope (derived from human type IV collagen) is dependent upon differential major histocompatibility complex (MHC) class II gene ex- pression 9. TH1 responses were ob- served in H-2 s mice, while TH2 re- sponses occurred in H-2 b mice. In F, mice, TH1 responses dominate, but TH2 responses could be observed if animals were treated with anti-H-2 S antibodies. One interpretation of these data is that quantitative differ- ences in MHC class II binding of antigen may influence the nature of the T-cell response.

At present, data from several lab- oratories indicate that cytokines, in particular IFN-y and IL-4, are criti- cal factors in the differentiation of T-cell subsets. In vitro, IFN-y aug- ments the development of TH1 cells, whereas IL-4 promotes TH2-cell dif- ferentiation (F. Fitch; S. Swain, La Jolla; and Ref. 10). In Leishmania major infection, anti-IFN-y treat- ment of resistant mice led to the development of a TH2 response after challenge infection, while adminis- tration of IFN-y with parasites pro- moted TH1 responses in BALB/c mice (P. Scott, Philadelphia). In con- trast, anti-lL-4 treatment reversed the susceptibility of BALB/c mice, and these animals demonstrated TH1 responses (R. Locksley, San Francisco), while administration of IL-4 to resistant mice augmented TH2 responses (R. Coffman, Palo Alto). Experiments in severe com- bined immunodeficient (SCID) mice further supported a critical role for these two cytokines in the differen- tiation of TH-ce]l subsets. Unexpec- tedly, normal BALB/c T cells transfer resistance to SCID mice. However, R. Coffman found that the develop- ment of resistance as well as T . I cells in these recipients was reversed when the animals were treated with anti-IFN-y monoclonal antibody, and suggests that high endogenous

Table 1. The potential role of T-cell subsets in infectious disease

Pathogen T-cell subsets a Suggested function b

Bacteria Listeria monocytogenes CD8-TH1 Protection Mycobacterium tuberculosis/ TH1-CD8 Protection

M. bovis Mycobacterium leprae TH1--TH2 (CD8) Protection-susceptibility

Protozoa Leishmania major TH1--TH2 Protection-exacerbation Trypanosoma cruzi CD8 Protection Plasmodium: hepatocyte CD8-TH1 Protection

erythrocyte TH1--TH2 Protection cerebral TH1 Immunopathology

Toxoplasma gondii CD8-TH1 Protection Theileria parva CD8 Protection

Metazoa Heligmosomoides polygyrus TH2 Protection Nippostrongylus brasiliensis TH2 Trichinella spiralis TH1--TH 2 Protection-susceptibility Schistosoma mansoni TH1--TH2 Protection-immunopathology

(murine) Toxocara canis TH2 -- Onchocerca volvulus TH2 -- Wucheria bancrofti TH2 --

Viruses Cytomegalovirus CD8-TH1 Protection Influenza virus CD8-TH1 Protection Measles virus TH1 Protection Murine leukemia (MAIDS) TH1--TH2 HIV TH1--TH 2 Protection-exacerbation

apredominant T-cell subset based upon cell depletion experiments, cell transfers or inferred from cytokine levels observed during infection. bin many cases direct evidence for in vivo function is not available.

levels of IFN-y, possibly produced by natural killer (NK) cells, are re- sponsible for the preferential de- velopment of TH1 cells in these recipients. The converse experiment was performed by R. Locksley, who found that while a TH2 T-cell line transferred enhanced susceptibility to SCID mice, mice that were also treated with anti-IL-4 antibodies de- veloped TH1 responses, and could control their infection.

IL-10 inhibits Tt~l cytokine pro- duction, apparently via alterations in macrophage antigen-presenting cell function (M. Howard, Palo Alto), and is a likely candidate for another cytokine that regulates T-cell subsets in vivo ~1. In this re- gard, significant IL-10 levels were observed during Nippostrongylus brasiliensis infections (T. Moss- man), as well as in routine

schistosomiasis during the time when TH1 responses were down- modulated (E. Pearce).

The cytokines that regulate CD4 ~ cells may come from a variety of sources. R. Seder (Bethesda) re- ported that the non-T, non-B FceR + cells known to produce high levels of IL-4 (Ref. 12) are most likely basophils, while IL-10, in addition to being produced by TH2 cells, is made by macrophages and B cells (M. Howard). It was also reported that IFN-cq which can be produced by many different cell types, en- hances TH1 responses in a manner similar to that reported for IFN-',/ (P. Scott).

The role of CD8 + cells in protective immune responses

CD8 + cells play a central role in anti-viral immunity and it is

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becoming increasingly clear that CD8 + T cells are also of major importance in many bacterial and protozoan infections. This has been shown most convincingly for Listeria monocytogenes but also for Myco- bacterium tuberculosis (M. bovis), Toxoplasma gondii, Plasmodium spp, Theileria parvum and Try- panosoma cruzi 13 (S. Kaufmann; J. Remington, Palo Alto; A. Sher; R. Nussenzweig, New York; S. Hoff- man, Bethesda; W. Morrison, North Newbury; R. Tarleton, Athens); even in leishmaniasis, CD8 + T cells contribute to protection (J. Louis, Lausanne). Most of the evidence comes from animal systems and the role of CD8 + cells in human infec- tions is less clear. However, CD8 + T-cell clones reactive against the circumsporozoite protein of P. falciparum were recently isolated from volunteers immunized with ir- radiated P. falciparum (S. Hoffman).

The mechanism by which bac- terial and protozoan antigens enter the class I pathway for presentation to CD8 + cells remains unclear, since these pathogens enter the endosomal compartment by phagocytosis. However, in the case of L. mono- cytogenes, a likely explanation can now be given. A major virulence factor of L. monocytogenes is list- eriolysin, which promotes evasion from the endosome into the cytoplasm, and deletion mutants lacking listeriolysin are avirulent (D. Portnoy, Philadelphia). This evasion mechanism probably allows contact between listerial antigens and the class I pathway. However, it is less clear how antigens from organisms that do not enter the cytoplasm are presented by class I. Perhaps during chronic infections the endosomal compartment becomes leaky and some antigens enter the cytoplasm.

Most CD8 + T cells express cyto- lytic activities and produce IFN-y, and it is not yet fully clear which of these functions are essential for protection - even in viral infections this prob- lem has not been solved satisfac- torily. However, experiments in the influenza system are consistent with cytolytic functions being protective, while in murine toxoplasmosis IFN-y production seems to be more relevant (T. Braciale, St. Louis; J. Remington).

As CD8 + T cells have entered the

stage in bacterial and protozoan in- fections, so CD4 + T cells have in

viral infections. In measles infections of rats, CD4 +, rather than CD8 ÷, T cells are crucial for protection (U. Liebert, Wurzburg). In CMV infec- tion, CD4 + T cells are not required as long as CD8 + T cells are present; however, after CD8 + T-cell de- pletion, CD4 + T cells can compen- sate for this defect and play a protective role (U. Kozinowski). Similarly, [32-microglobulin-deficient mice, which lack CD8 + T ceils, can control influenza infection but suc- cumb when CD4 + T cells are de- pleted (P. Doherty, Memphis). In contrast, 132-microglobulin-deficient mice suffer severely from T. cruzi infection (R. Tarleton). Protective CD4 ÷ T cells in murine influenza, malaria and listeriosis produce IFN-y and are cytolytic, and hence resemble Twl cells (T. Braciale; R. Nussensweig; S. Kaufmann). TH2 cells represent a minor fraction in influenza infection, and are not pro- tective upon adoptive transfer (T. Braciale).

The finding that CD8 + T cells are induced by certain nonviral patho- gens and are essential for protection has major consequences for vaccine design. First, vaccines against such pathogens must be capable of intro- ducing antigens into the class I pathway. Second, recombinant bac- teria can be used as carriers for vaccination against viral infection. Indeed, recombinant L. mono- cytogenes and M. bovis BCG have been shown to induce cytolytic CD8 + T cells with specificity for the recombinant protein (Y. Patterson, Philadelphia; R. Young, Boston). In another approach, transfectants ex- pressing a 140 kDa sporozoite sur- face antigen and the circumsporo- zoite protein induce full protection against murine malaria which de- pends on CD8 ÷ T cells (S. Hoffman).

y~ T cells and anti-bacterial immunity

Human y8 T cells are markedly expanded by in vitro stimulation with mycobacteria and other bac- teria, and they have been identified in leprosy lesions (S. Kaufmann; R. Modlin). Such y8 T cells produce tumor necrosis factor (TNF) and IFN-y and express cytolytic activity; hence, they resemble their c43 T-cell

counterparts (CD8 + T cells, TH1 cells). Furthermore, in vivo priming of 0~/3 T cells with mycobacteria is diminished in the absence of y~ T cells, as assessed in SCID mice recon- stituted with purified T-cell popu- lations (D. Pardoll, Baltimore). y~ T cells have also been identified in the lung of influenza-virus-infected mice (P. Doherty), indicating that they participate in immunity to different types of pathogen. Yet, the precise role of y8 T cells in the immune response to bacterial pathogens re- mains elusive.

Phillip Scott is at the Dept of Patho- biology, Veterinary School of Medi- cine, University of Pennsylvania, 3800 Spruce St, Philadelphia, PA., USA and Stefan H.E. Kaufmann is at the Dept of Immunology, University of Ulm, Albert-Einstein-AIlee 11, D-7900 Ulm, FR G.

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