Characterization of the CD154-positive and CD40-positive cellular subsets required for pathogenesis in retrovirus-induced murine immunodeficiency

Genetically susceptible C57BL/6 (B6) mice that are infected with the LP-BM5 isolate of murine retroviruses develop profound splenomegaly, lymphadenopathy, hypergammaglobulinemia, terminal B-cell lymphomas, and an immunodeficiency state bearing many similarities to the pathologies seen in AIDS. Because of these similarities, this syndrome has been called murine AIDS (MAIDS). We have previously shown that CD154 (CD40 ligand)-CD40 molecular interactions are required both for the initiation and progression of MAIDS. Thus, in vivo anti-CD154 monoclonal antibody (MAb) treatment inhibited MAIDS symptoms in LP-BM5-infected wild-type mice when either a short course of anti-CD154 MAb treatment was started on the day of infection or a course was initiated 3 to 4 weeks after LP-BM5 administration, after disease was established. Here, we further characterize this required CD154-CD40 interaction by a series of adoptive transfer experiments designed to elucidate which cellular subsets must express CD154 or CD40 for LP-BM5 to induce MAIDS. Specifically with regard to CD154 expression, MAIDS-insusceptible B6 nude mice reconstituted with highly purified CD4+ T cells from wild-type, but not from CD154 knockout, B6 donors displayed clear MAIDS after LP-BM5 infection. In contrast, nude B6 recipients that received CD8+ T cells from wild-type B6 donors did not develop MAIDS after LP-BM5 infection. B6 CD40 knockout mice, which are also relatively resistant to LP-BM5-induced MAIDS, became susceptible to LP-BM5-induced disease after reconstitution with highly purified wild-type B cells but not after receiving purified wild-type dendritic cells (DC) or a combined CD40+ population composed of DC and macrophages obtained from B6 SCID mouse donors. Based on these and other experiments, we thus conclude that the cellular basis for the requirement for CD154-CD40 interactions for MAIDS induction and progression can be accounted for by CD154 expression on CD4+ T cells and CD40 expression on B cells.     

Impaired calcium mobilization in CD4+ and CD8+ T cells in a retrovirus-induced immunodeficiency syndrome, murine AIDS.

After infection with LP-BM5 murine leukemia viruses, susceptible strains of mice develop a severe and progressive immunodeficiency disease, termed murine AIDS (MAIDS), features of which include markedly impaired T cell response to mitogens or specific Ag stimulation and decreased production of IL-2. Since an elevation of intracellular calcium concentration resulting from binding of Ag to the TCR is associated with IL-2 production, T cells from mice either uninfected or infected with LP-BM5 murine leukemia viruses were examined by a calcium mobilization assay. Both CD4+ and CD8+ T cells from infected mice manifested impaired calcium mobilization responses upon in vitro stimulation with anti-CD3 mAb or Con A. The abnormalities appeared early after virus inoculation and showed no difference in time course between subsets of T cells. Frequencies of prestimulation calcium-positive cells among both CD4+ and CD8+ cells in mice with MAIDS were significantly higher than those for uninfected mice. These abnormalities were associated with presence of the MAIDS-inducing defective virus genome, but were not induced by infection of mice genetically resistant to development of MAIDS or with nonpathogenic helper murine leukemia virus, a virus component that induces high spontaneous proliferation of T cells, even in MAIDS-resistant mice.     

T cells cause acute immunopathology and are required for long-term survival in mouse adenovirus type 1-induced encephalomyelitis

Infection of adult C57BL/6 (B6) mice with mouse adenovirus type 1 (MAV-1) results in dose-dependent encephalomyelitis. Utilizing immunodeficient mice, we analyzed the roles of T cells, T-cell subsets, and T-cell-related functions in MAV-1-induced encephalomyelitis. T cells, major histocompatibility complex (MHC) class I, and perforin contributed to acute disease signs at 8 days postinfection (p.i.). Acute MAV-1-induced encephalomyelitis was absent in mice lacking T cells and in mice lacking perforin. Mice lacking alpha/beta T cells had higher levels of infectious MAV-1 at 8 days, 21 days, and 12 weeks p.i., and these mice succumbed to MAV-1-induced encephalomyelitis at 9 to 16 weeks p.i. Thus, alpha/beta T cells were required for clearance of MAV-1. MAV-1 was cleared in mice lacking perforin, MHC class I or II, CD4+ T cells, or CD8+ T cells. Our results are consistent with a model in which either CD8+ or CD4+ T cells are sufficient for clearance of MAV-1. Furthermore, perforin contributed to MAV-1 disease but not viral clearance. We have established two critical roles for T cells in MAV-1-induced encephalomyelitis. T cells caused acute immunopathology and were required for long-term host survival of MAV-1 infection.     

Adaptive immunity and enhanced CD8+ T cell response to Listeria monocytogenes in the absence of perforin and IFN-gamma

Single Ag-specific CD8+ T cells from IFN-gamma-deficient (GKO) or perforin-deficient (PKO) mice provide substantial immunity against murine infection with Listeria monocytogenes. To address the potential for redundancy between perforin and IFN-gamma as CD8+ T cell effector mechanisms, we generated perforin/IFN-gamma (PKO/GKO) double-deficient mice. PKO/GKO-derived CD8+ T cells specific for the immunodominant listeriolysin O (LLO91-99) epitope provide immunity to LM infection similar to that provided by Ag-matched wild-type (WT) CD8+ T cells in the liver but reduced in the spleen. Strikingly, polyclonal CD8+ T cells from immunized PKO/GKO mice were approximately 100-fold more potent in reducing bacterial numbers than the same number of polyclonal CD8+ T cells from immunized WT mice. This result is probably quantitative, because the frequency of the CD8+ T cell response against the immunodominant LLO91-99 epitope is >4.5-fold higher in PKO/GKO mice than WT mice at 7 days after identical immunizations. Moreover, PKO/GKO mice can be immunized by a single infection with attenuated Listeria to resist >80,000-fold higher challenges with virulent organisms than naive PKO/GKO mice. These data demonstrate that neither perforin nor IFN-gamma is required for the development or expression of adaptive immunity to LM. In addition, the results suggest the potential for perforin and IFN-gamma to regulate the magnitude of the CD8+ T cell response to infection.     

Analysis of role of CD8+ T cells in resistance to murine AIDS in A/J mice.

The mixture of retroviruses termed LP-BM5 murine leukemia virus (MuLV) contains a replication-defective genome (BM5def), the crucial element for induction of murine AIDS (MAIDS), as well as helper B-tropic ecotropic and mink cell focus-forming MuLV. Among Fv-1b mouse strains, C57BL mice are sensitive to infection by these viruses and to development of MAIDS, but A/J mice are highly resistant to all viral components and to induction of disease. Inasmuch as previous genetic studies indicated a major role in susceptibility for the H-2D locus within the MHC, the effect of CD8+ T cells in A/J resistance to MAIDS was analyzed by depletion of this subset using mAb. A/J mice treated with anti-CD8 mAb beginning soon after inoculation with LP-BM5 MuLV developed disease within 5 wk after virus inoculation. Histopathologic and flow cytometry alteration of tissues and cells from the mAb-treated mice were identical to those seen in virus-infected MAIDS-sensitive strains, and assays for MuLV demonstrated high-level expression of ecotropic MuLV and integration of BM5def. Parallel studies of A/J mice treated with anti-CD4 mAb after infection revealed enhanced expression of ecotropic MuLV but no integration of BM5def, and no signs of MAIDS were detected. These observations indicate that CD8+ T cells are critical in the resistance of A/J mice to LP-BM5 MuLV replication and development of disease and suggest that CD4+ T cells play a role in regulation of ecotropic virus replication.     

CD8+ T cells require perforin to clear West Nile virus from infected neurons

Injury to neurons after West Nile virus (WNV) infection is believed to occur because of viral and host immune-mediated effects. Previously, we demonstrated that CD8+ T cells are required for the resolution of WNV infection in the central nervous system (CNS). CD8+ T cells can control infection by producing antiviral cytokines (e.g., gamma interferon or tumor necrosis factor alpha) or by triggering death of infected cells through perforin- or Fas ligand-dependent pathways. Here, we directly evaluated the role of perforin in controlling infection of a lineage I New York isolate of WNV in mice. A genetic deficiency of perforin molecules resulted in higher viral burden in the CNS and increased mortality after WNV infection. In the few perforin-deficient mice that survived initial challenge, viral persistence was observed in the CNS for several weeks. CD8+ T cells required perforin to control WNV infection as adoptive transfer of WNV-primed wild-type but not perforin-deficient CD8+ T cells greatly reduced infection in the brain and spinal cord and enhanced survival of CD8-deficient mice. Analogous results were obtained when wild-type or perforin-deficient CD8+ T cells were added to congenic primary cortical neuron cultures. Taken together, our data suggest that despite the risk of immunopathogenesis, CD8+ T cells use a perforin-dependent mechanism to clear WNV from infected neurons.     

Induction of CD4 T cell changes in murine AIDS is dependent on costimulation and involves a dysregulation of homeostasis

Strong CD4 T cell activation and proliferation are seen in susceptible mice infected with the murine retroviral inoculum, LP-BM5, which produces an immunodeficiency syndrome called murine AIDS (MAIDS). We developed a short term adoptive transfer model of MAIDS to examine the requirements for the CD4 T cell response. Naive CD4 T cells from uninfected donors responded quickly after adoptive transfer into MAIDS-infected hosts, becoming activated and proliferating within several days. Using blocking mAbs to costimulatory ligands and CD4 T cells deficient in expression of their receptors, we found that the CD4 T cell response requires CD28:B7.1/B7.2 interactions, but not CTLA4 or CD40-CD40 ligand interactions. Naive CD4 T cells did not respond in H-2M-deficient mice with MAIDS, suggesting that disease requires recognition of self peptide-MHC complexes. The self MHC-dependent division and accumulation of large numbers of CD4 T cells suggest that MAIDS involves a disruption of the balance of homeostatic signals. Supporting this hypothesis, CD4 T cells from mice with MAIDS failed to regulate the homeostatic division of naive CD4 T cells in a cotransfer model. Thus, a combination of up-regulation of costimulatory ligands and disruption of homeostatic control may be responsible for CD4 lymphoproliferation in MAIDS.     

Zidovudine (azido dideoxythymidine) inhibits characteristic early alterations of lymphoid cell populations in retrovirus-induced murine AIDS

Using flow cytometry technology and multiparameter analyses, we report early and characteristic alterations in lymphoid cell profile in spleen and lymph nodes due to LP-BM5 retrovirus disease (murine AIDS (MAIDS)) and the effect of azido dideoxythymidine, a nucleoside inhibitor, on these changes. MAIDS has been characterized by rapid and profound lymphoproliferation accompanied by hypergammaglobulinemia and immunosuppression. As early as 2 wk postinfection, there is a selective depletion of CD8+ cells whereas the total number of CD4+ cells increases throughout the first 8 wk of infection although the frequency is relatively stable. These population changes were partially delayed by oral AZT therapy for 6 wk postinfection. Ly-6C (AL-21) is expressed on roughly 50% of CD4+ and CD8+ cells in C57BL/6 mice. In MAIDS, the residual population of CD8+ cells is primarily Ly-6C+. The CD4+ cells have a transient increase in ratio of Ly-6C+/Ly-6C- cells at 2 wk postinfection but by 6 wk are primarily Ly-6C-. There was an increase in both the total number and percentage of Mac 1+ cells and a selective depletion of certain splenic B cell subpopulations. Azido dideoxythymidine delays these early population changes.     

The role of IL-12 in the control of MCMV is fundamentally different in mice with a retroviral immunodeficiency syndrome (MAIDS)

The present study investigates the susceptibility of C57BL mice exhibiting T cell immunodeficiency and lymphadenopathy induced by LP-BM5 murine leukaemia virus (MAIDS) to murine cytomegalovirus (MCMV). Treatment of normal (M-) mice with anti-IL-12 increased the contribution of IgG1 to the hypergammaglobulinaemia induced by MCMV, consistent with a shift towards a Th2 phenotype. This impaired control of early MCMV replication in the liver, with little effect in the spleen. Control of hepatic infection correlated with a vigorous splenic NK cytotoxic response in a subgroup of IL-12-depleted M- mice that remained healthy, while others became moribund. Mortality in IL-12-depleted MAIDS (M+) mice given MCMV was ultimately greater than in M- controls, but was delayed despite high levels of MCMV in the liver. IL-12 was required for optimal control of MCMV replication in M+ mice. This may involve cytotoxic activity because similar levels of infection were seen in bg/bg M+ mice, where the beige mutation impairs the formation of cytotoxic granules. Hence the ability of M+ mice to tolerate high titres of MCMV during acute infection may enable innate cytotoxic responses to clear MCMV. Interleukin-12 depletion of M- mice also increased salivary gland MCMV titres and depressed delayed-type hypersensitivity responses to MCMV antigen, normally mediated by CD4+ T cells. These changes were not observed in IL-12-depleted M+ mice.     

Role of IP-10/CXCL10 in the progression of pancreatitis-like injury in mice after murine retroviral infection

Exocrinopathy and pancreatitis-like injury were developed in C57BL/6 (B6) mice infected with LP-BM5 murine leukemia virus, which is known to induce murine acquired immunodeficiency syndrome (MAIDS). The role of chemokines, especially CXCL10/interferon (IFN)-gamma-inducible protein 10 (IP-10), a chemokine to attract CXCR3+ T helper 1-type CD4+ T cells, has not been investigated thoroughly in the pathogenesis of pancreatitis. B6 mice were inoculated intraperitoneally with LP-BM5 and then injected every week with either an antibody against IP-10 or a control antibody. Eight weeks after infection, we analyzed the effect of IP-10 neutralization. Anti-IP-10 antibody treatment did not change the generalized lymphadenopathy and hepatosplenomegaly of mice with MAIDS. The treatment significantly reduced the number of IP-10- and CXCR3-positive cells in the mesenteric lymph nodes (mLNs) but not the phenotypes and gross numbers of cells. In contrast, IP-10 neutralization reduced the number of mononuclear cells infiltrating into the pancreas. Anti-IP-10 antibody treatment did not change the numbers of IFN-gamma+ and IL10+ cells in the mLN but significantly reduced their numbers, especially IFN-gamma+ and IL-10+ CD4+ T cells and IFN-gamma+ Mac-1+ cells, in the pancreas. IP-10 neutralization ameliorated the pancreatic lesions of mice with MAIDS probably by blocking the cellular infiltration of CD4+ T cells and IFN-gamma+ Mac-1+ cells into the pancreas at least at 8 wk after infection, suggesting that IP-10 and these cells might play a key role in the development of chronic autoimmune pancreatitis.     

CD8+ CTL from lungs of Mycobacterium tuberculosis-infected mice express perforin in vivo and lyse infected macrophages

CD8+ T lymphocytes have been implicated in the protective immune response against human and murine tuberculosis. However, the functional role that this cell subset plays during the resolution of infection remains controversial. In this study, we demonstrate the presence of Mycobacterium tuberculosis-specific CD8+ CTL in the lungs and lung-draining lymph nodes of mice infected with M. tuberculosis via the aerosol or i.v. route. These cells expressed perforin in vivo and specifically recognized and lysed M. tuberculosis-infected macrophages in a perforin-dependent manner after a short period of in vitro restimulation. The efficiency of lysis of infected macrophages was dependent upon the time allowed for interaction between macrophage and M. tuberculosis bacilli. Recognition of infected targets by CD8+ CTL was beta 2-microglobulin and MHC class I dependent and was not CD1d restricted. The presented data indicate that CD8+ T cells contribute to the protective immune response during M. tuberculosis infection by exerting cytotoxic function and lysing infected macrophages.     

Apoptosis by CD95 (Fas)-dependent and -independent mechanisms in Peyer's patch lymphocytes in murine retrovirus-induced immunodeficiency syndrome.

CD95 (Fas)/CD95 ligand (CD95 L)-mediated apoptosis is thought to be involved in the delayed progression of murine AIDS (MAIDS) induced by LP-BM5 murine leukemia virus (MuLV). We show evidence of apoptosis in lymphocytes of Peyer's patches (PP) at the early stage of MAIDS. Both T and B cells in PP expressed CD95 at the early stage of MAIDS and decreased in number thereafter. The decrease in T cells was not evident in CD95-mutated lpr mice with MAIDS, suggesting that CD95/CD95 L interaction is involved in the apoptosis of T cells in PP during the course of MAIDS. On the other hand, the number of B cells was also decreased in PP of lpr mice with MAIDS. The proliferative ability of B cells in PP of MAIDS mice in response to immunoglobulin M cross-linking or lipopolysaccharide was severely impaired, while the B cells normally proliferated in response to anti-CD40 monoclonal antibody. These findings imply that aberrantly activated B cells in PP undergo apoptosis independently of the CD95/CD95 L system during the course of infection with MAIDS virus.     

Delayed clearance of Sendai virus in mice lacking class I MHC-restricted CD8+ T cells.

The role and interdependence of CD8+ and CD4+ alpha beta-T cells in the acute response after respiratory infection with the murine parainfluenza type 1 virus, Sendai virus, has been analyzed for H-2b mice. Enrichment of CD8+ virus-specific CTL effectors in the lungs of immunologically intact C57BL/6 animals coincided with the clearance of the virus from this site by day 10 after infection. Removal of the CD4+ T cells by in vivo mAb treatment did not affect appreciably either the recruitment of CD8+ T cells to the infected lung, or their development into virus-specific cytotoxic effectors. In contrast, depletion of the CD8+ subset delayed virus clearance, although most mice survived the infection. Transgenic H-2b F3 mice homozygous (-/-) for a beta 2 microglobulin (beta 2-m) gene disruption, which lack both class I MHC glycoproteins and mature CD8+ alpha beta-T cells, showed a comparable, delayed clearance of Sendai virus from the lung. Virus-specific, class II MHC-restricted CTL were demonstrated in both freshly isolated bronchoalveolar lavage populations and cultured lymph node and spleen tissue from the beta 2-m (-/-) transgenics. Treatment of the beta 2-m (-/-) mice with the mAb to CD4 led to delayed virus clearance and death, which was also the case for normal mice that were depleted simultaneously of the CD4+ and CD8+ subsets. These results indicate that, although classical class I MHC-restricted CD8+ cytotoxic T cells normally play a dominant role in the recovery of mice acutely infected with Sendai virus, alternative mechanisms involving CD4+ T cells exist and can compensate, in time, for the loss of CD8+ T cell function.     

CD28-B7 Costimulatory Blockade by CTLA4Ig Delays the Development of Retrovirus-Induced Murine AIDS

Mouse AIDS (MAIDS) induced in C57BL/6 mice by infection with a replication-defective retrovirus (Du5H) combines extensive lymphoproliferation and profound immunodeficiency. Although B cells are the main target of viral infection, recent research has focused on CD4⁺ T cells, the activation of which is a key event in MAIDS induction and progression. A preliminary observation of increased expression of B7 molecules on B cells in MAIDS prompted us to address the possible involvement of the CD28/B7 costimulatory pathway in MAIDS. Mice infected with the MAIDS-inducing viral preparation were treated with murine fusion protein CTLA4Ig (3 × 50 μg/week given intraperitoneally), a competitive inhibitor of physiological CD28-B7 interactions. In CTLA4Ig-treated animals, the onset of the disease was delayed, lymphoproliferation progressed at a much slower rate than in untreated mice, and the loss of in vitro responsiveness to mitogens was reduced. Relative expression of Du5H did not differ between treated and untreated animals. These results suggest that the CD28/B7 costimulatory pathway contributes to MAIDS development.     

CD8+ T cells can mediate almost complete short-term and partial long-term immunity to rotavirus in mice.

We have recently shown that CD8+ T cells mediate clearance of rotavirus infection in mice. B-cell-deficient J(H)D knockout (-/-) mice depleted of CD8+ T cells become chronically infected with murine rotavirus, and beta2 microglobulin -/- and other mice depleted of CD8+ T cells have a 1- to 4-day delay in clearance of primary rotavirus infection. A role for CD8+ T cells in protection from reinfection with rotavirus was suggested by these studies, because J(H)D -/- mice rechallenged 6 to 8 weeks after primary infection shed smaller quantities of viral antigen and for fewer days than naive mice. Here we show that 8, 11, 13, and 18 days after primary infection the J(H)D -/- mice are almost completely resistant to reinfection and that they are still partially protected from reinfection 6 weeks, 5 months, and 8 months after primary infection. Protection against reinfection was dependent on CD8+ T cells, since J(H)D -/- mice depleted of CD8+ T cells by administration of an anti-CD8 monoclonal antibody became chronically infected with rotavirus upon rechallenge 13 days, 18 days, 6 weeks, and 5 months after primary infection. Thus, CD8+ T cells can actively mediate almost complete short-term and partial long-term protection from reinfection.     

CD8+ T lymphocytes are not required for murine resistance to human filarial parasites.

Mice are resistant to the establishment of infection with the nematode parasite Brugia malayi, an etiologic agent of human lymphatic filariasis. We have recently shown that T and B lymphocyte-deficient C.B.-17 scid/scid mice are permissive for infection with this parasite, whereas coisogenic C.B.-17+/+ mice are resistant. This observation suggests that T and B lymphocytes that comprise the antigen-specific immune system orchestrate murine resistance to B. malayi. In order to define the component of the antigen-specific immune response that is responsible for this resistance, we have tested the susceptibility of beta 2M-/- mice to infection with B. malayi L3 larvae. These mice are homozygous for insertional disruption of their B2m genes, which encode beta 2-microglobulin, the small subunit of the major histocompatibility (MHC) antigens. They do not express beta 2-microglobulin and, as a consequence, fail to express the class I major histocompatibility antigens, and they do not develop the CD8+ class I MHC-restricted cytotoxic T cell subset. We find that these mice are completely resistant to B. malayi, indicating that the CD8+ T lymphocyte subset is not an obligate requirement for murine resistance to human filarial parasites.     

Regulation of human cell engraftment and development of EBV-related lymphoproliferative disorders in Hu-PBL-scid mice

Human PBMC engraft in mice homozygous for the severe combined immunodeficiency (Prkdcscid) mutation (Hu-PBL-scid mice). Hu-PBL-NOD-scid mice generate 5- to 10-fold higher levels of human cells than do Hu-PBL-C.B-17-scid mice, and Hu-PBL-NOD-scid beta2-microglobulin-null (NOD-scid-B2mnull) mice support even higher levels of engraftment, particularly CD4+ T cells. The basis for increased engraftment of human PBMC and the functional capabilities of these cells in NOD-scid and NOD-scid-B2mnull mice are unknown. We now report that human cell proliferation in NOD-scid mice increased after in vivo depletion of NK cells. Human cell engraftment depended on CD4+ cells and required CD40-CD154 interaction, but engrafted CD4+ cells rapidly became nonresponsive to anti-CD3 Ab stimulation. Depletion of human CD8+ cells led to increased human CD4+ and CD20+ cell engraftment and increased levels of human Ig. We further document that Hu-PBL-NOD-scid mice are resistant to development of human EBV-related lymphoproliferative disorders. These disorders, however, develop rapidly following depletion of human CD8+ cells and are prevented by re-engraftment of CD8+ T cells. These data demonstrate that 1) murine NK cells regulate human cell engraftment in scid recipients; 2) human CD4+ cells are required for human CD8+ cell engraftment; and 3) once engrafted, human CD8+ cells regulate human CD4+ and CD20+ cell expansion, Ig levels, and outgrowth of EBV-related lymphoproliferative disorders. We propose that the Hu-PBL-NOD-scid model is suitable for the in vivo analysis of immunoregulatory interactions between human CD4+ and CD8+ cells.     

Cutting edge: antilisterial activity of CD8+ T cells derived from TNF-deficient and TNF/perforin double-deficient mice

The mechanisms by which CD8+ T cells mediate immunity against bacterial pathogens remain largely unknown. Perforin-dependent cytolysis plays a role, but is not required for CD8+ T cell-mediated immunity against Listeria monocytogenes. TNF is essential for CD8+ T cell immunity to L. monocytogenes, but the cellular source of TNF is undefined. TNF-deficient and TNF/perforin double-deficient mice were used to generate CD8+ T cells specific for an L. monocytogenes-derived Ag. Wild-type and TNF-deficient CD8+ T cells mediated antilisterial immunity in wild-type but not TNF-deficient host mice, revealing that CD8+ T cell-derived TNF is not required for CD8+ T cell-mediated antilisterial immunity, but demonstrating a role for TNF derived from other cell types. TNF/perforin double-deficient CD8+ T cells mediated antilisterial immunity in the liver, but not in the spleen, of wild-type recipient mice, suggesting that perforin-independent immunity in the spleen requires CD8+ T cell-derived TNF.     

Gamma delta T cell-deficient mice have a down-regulated CD8+ T cell immune response against Encephalitozoon cuniculi infection

Gamma(delta) T cells have been reported to play an essential effector role during the early immune response against a wide variety of infectious agents. Recent studies have suggested that the gamma(delta) T cell subtype may also be important for the induction of adaptive immune response against certain microbial pathogens. In the present study, an early increase of gamma(delta) T cells during murine infection with Encephalitozoon cuniculi, an intracellular parasite, was observed. The role of gamma(delta) T cells against E. cuniculi infection was further evaluated by using gene-knockout mice. Mice lacking gamma(delta) T cells were susceptible to E. cuniculi infection at high challenge doses. The reduced resistance of delta(-/-) mice was attributed to a down-regulated CD8+ immune response. Compared with parental wild-type animals, suboptimal Ag-specific CD8+ T cell immunity against E. cuniculi infection was noted in delta(-/-) mice. The splenocytes from infected knockout mice exhibited a lower frequency of Ag-specific CD8+ T cells. Moreover, adoptive transfer of immune TCR(alpha)beta+ CD8+ cells from the delta(-/-) mice failed to protect naive CD8(-/-) mice against a lethal E. cuniculi challenge. Our studies suggest that gamma(delta) T cells, due to their ability to produce cytokines, are important for the optimal priming of CD8+ T cell immunity against E. cuniculi infection. This is the first evidence of a parasitic infection in which down-regulation of CD8+ T cell immune response in the absence of gamma(delta) T cells has been demonstrated.     

The role of CD4 T cells in the pathogenesis of murine AIDS

LP-BM5, a retroviral isolate, induces a disease featuring retrovirus-induced immunodeficiency, designated murine AIDS (MAIDS). Many of the features of the LP-BM5-induced syndrome are shared with human immunodeficiency virus-induced disease. For example, CD4 T cells are critical to the development of MAIDS. In vivo depletion of CD4 T cells before LP-BM5 infection rendered genetically susceptible B6 mice MAIDS resistant. Similarly, MAIDS did not develop in B6.nude mice. However, if reconstituted with CD4 T cells, B6.nude mice develop full-blown MAIDS. Our laboratory has shown that the interaction of B and CD4 T cells that is central to MAIDS pathogenesis requires ligation of CD154 on CD4 T cells with CD40 on B cells. However, it is not clear which additional characteristics of the phenotypically and functionally heterogeneous CD4 T-cell compartment are required. Here, in vivo adoptive transfer experiments using B6.nude recipients are employed to compare the pathogenic abilities of CD4 T-cell subsets defined on the basis of cell surface phenotypic or functional differences. Th1 and Th2 CD4 T cells equally supported MAIDS induction. The rare Thy1.2(-) CD4 subset that expands upon LP-BM5 infection was not necessary for MAIDS. Interestingly, CD45RB(low) CD4 T cells supported significantly less disease than CD45RB(high) CD4 T cells. Because the decreased MAIDS pathogenesis could not be attributed to inhibition by CD45RB(low) CD25(+) natural T-regulatory cells, an intrinsic property of the CD45RB(low) cells appeared responsible. Similarly, there was no evidence that natural T-regulatory cells played a role in LP-BM5-induced pathogenesis in the context of the intact CD4 T-cell population.