Deficient Humoral Responses Underlie Susceptibility to Toxoplasma gondii in CD4-Deficient Mice

Resistance to infection with Toxoplasma gondii was studied in mice lacking CD4 expression. Such mice developed more brain cysts and survived for a shorter time than did wild-type controls after peroral infection with ME49 cysts. After immunization with the ts-4 strain of T. gondii, CD4-deficient mice exhibited impaired resistance to a challenge infection with virulent RH tachyzoites. Thus, deficient CD4 expression increases the susceptibility of mice to a primary peroral T. gondii infection with cysts and impairs their ability to be successfully vaccinated. CD8(+) T cells from blood or spleens of Toxoplasma-infected, CD4-deficient mice expressed markers of activation at frequencies similar to those of infected wild-type mice. Production of IFN-gamma in vitro was moderately depressed, and levels of Toxoplasma-specific immunoglobulin G2a in serum were substantially lower than in wild-type mice. Administration of Toxoplasma-immune serum to ts-4-vaccinated CD4-deficient mice significantly improved their resistance to RH challenge. Also, the survival of CD4-deficient mice chronically infected with ME49 was significantly prolonged by administration of immune serum. These results demonstrate that in addition to CD8(+) T cells and IFN-gamma, which are known to be critical for resistance, CD4(+) cells also contribute significantly to protection against chronic T. gondii infections and against challenge infections with highly virulent tachyzoites in immunized mice via their role as helper cells for production of isotype-switched antibodies.     

Interactions between murine AIDS (MAIDS) and toxoplasmosis in co-infected mice.

We coinfected C57B1/6 mice with LP-BM5 murine leukaemia viruses, responsible for murine AIDS (MAIDS), and an avirulent strain of Toxoplasma gondii. Virus-infected mice were infected perorally on day 30 with 10 cysts of T. gondii, and T. gondii-infected mice were challenged with LP-BM5 on day 20, 30 or 60 after parasite inoculation. Uninfected and singly infected mice were used as controls. The kinetics of parasite burden in blood, lungs and brain, together with blood lymphocyte subsets, and spleen and lymph node weights, were serially determined in each group of mice. The kinetics of parasite counts in mice infected by LP-BM5 then by T. gondii were similar to those in mice infected by T. gondii only, except for lung counts, which reached higher values than in animals infected with T. gondii alone, then fell and re-increased until the end of the experiment. The only significant change in parasite burdens when mice were first infected by T. gondii and then by LP-BM5, compared with T. gondii controls, was an increase in lung counts in mice challenged with LP-BM5 20 days after T. gondii inoculation. Whatever the schedule of co-infection, the kinetics of lymphocyte subsets in co-infected mice differed from those in T. gondii- or LP-BM5-infected mice; in dually infected mice CD4+ and CD8+ cell counts were intermediate between values in mice singly infected by the parasite or the virus. Enlargement of spleen and lymph nodes, which is a major criterion of MAIDS progression, was significantly less marked in co-infected mice than in mice infected with LP-BM5 alone. These data point to cross-regulation of T. gondii and LP-BM5 infections, which results in increased susceptibility to T. gondii, and may alter the progression of MAIDS.     

Protective immunity in murine histoplasmosis: functional comparison of adoptively transferred T-cell clones and splenic T cells.

In this study, I examined whether a murine T-cell line and three clones that recognize Histoplasma capsulatum antigens in vitro could confer protection in vivo against a challenge of Histoplasma yeasts. C57BL/6 mice were each inoculated with 5 X 10(4) yeasts intravenously; 1 h later, 5 X 10(6) or 2 X 10(7) resting T cells were inoculated intravenously. At week 1 of infection, the T-cell line and all clones failed to reduce the number of H. capsulatum CFU in the spleens of mice compared with numbers in infected controls. Administration of recombinant interleukin 2 or cyclophosphamide to infected mice did not potentiate the functional activity in vivo of either the T-cell line or the clones. In contrast, inoculation with 2 X 10(7) CD4+ but not CD8+ cells isolated from the spleens of mice immunized with 10(6) viable yeast cells sharply diminished the number of CFU in the spleens of infected animals. Moreover, splenic CD4+ cells from immune mice transferred a delayed-type hypersensitivity response, whereas the T-cell line and clones did not. Injection Injection of an equal number of cloned T cells and CD8+ splenocytes from immune mice did not transfer resistance to infected mice. Additional studies were undertaken to determine if the ineffectiveness of cloned T cells was associated with a failure to migrate to and survive within spleens of infected mice. B6.PL Thy-1a/Cy mice, which are genetically identical to C57BL/6 mice except that T cells of the former bear Thy-1.1 rather than Thy-1.2, were inoculated with Histoplasma yeasts and then injected with immune CD4+ splenocytes or a T-cell clone. At days 1 and 7 of infection, virtually no Thy-1.2+ cells were detected in the spleens of infected mice given cloned T cells. However, the spleens of animals inoculated with immune CD4+ cells contained a small but significant (P less than 0.01) proportion of Thy-1.2+ cells at both day 1 and day 7 postinoculation of H. capsulatum. Thus, the failure of T-cell clones to transfer protection against H. capsulatum may be explained by defective trafficking or poor survival in vivo or both.     

Differential susceptibilities of mice genomically deleted of CD4 and CD8 to infections with Trypanosoma cruzi or Trypanosoma brucei.

The role of CD4+ and CD8+ T cells in the surveillance of Trypanosoma cruzi or Trypanosoma brucei brucei was studied in mice which lacked CD4 or CD8 molecules and which were generated by embryonic stem cell technology. Whereas wild-type mice infected with T. cruzi (Tulahuén strain) displayed low levels of parasitemia and no mortality, striking increases in parasite growth and mortality occurred in both CD8- and CD4- mice. On the contrary, CD8- and, to a lesser degree, CD4- mice showed enhanced resistance to T. b. brucei. T-cell-dependent immunoglobulin G-specific responses were produced in CD8- but not CD4- mice. Normal T-cell proliferative responses were measured in both CD4- and CD8- mice. Interleukin-4 production after concanavalin A or anti-CD3 monoclonal antibody stimulation was strikingly enhanced in CD8- but not CD4- spleen cells, whereas gamma interferon production was normal in both CD4- and CD8- spleen cells. Spleen and lymph node cells from CD8- (but not CD4-) mice at 20 days postinfection with T. cruzi had higher levels of interleukin-4 mRNA than the wild-type controls, as shown in a competitive polymerase chain reaction assay. On the other hand, CD4- (but not CD8-) mice at 20 days postinfection with T. cruzi had lower levels of gamma interferon mRNA than the wild-type mice.     

CD95 is required for the early control of parasite burden in the liver of Leishmania donovani-infected mice

In this study we show an increased incidence of T cell apoptosis in the liver and spleen of mice infected with Leishmania donovani. T cells from L. donovani-infected mice were found to be increasingly susceptible to CD95-mediated apoptosis in vitro, compared to controls. To test if suboptimal T cell function resulting from CD95-mediated apoptosis contributes to sustained parasite burden in L. donovani parasitized mice, B6.gld mice (lacking functional CD95 ligand) were infected with L. donovani. Surprisingly, at four different time points no difference in levels of T cell apoptosis in the spleen and liver was found between these mice and controls following intravenous delivery of L. donovani amastigotes, indicating that the CD95 / CD95L interaction is not essential for T cell apoptosis in the L. donovani-infected liver and spleen. However, B6.gld mice were increasingly susceptible to L. donovani infection, associated with less efficient granuloma formation in the liver and uncontrolled parasite growth in the spleen. Late in infection (day 56 post-infection), B6.gld mice had higher numbers of IFN-gamma-producing CD4(+) T cells in the liver and spleen, indicating a role for CD95 signaling in the homeostasis of this subset of cytokine-producing T cells in L. donovani-parasitized mice. Adoptive transfer of CD4(+) and CD8(+) T cells into recombinase activating gene 1 knockout (RAG-1(- / -)) recipients, revealed that CD95L expressed on CD4(+) T cells contributes to early control of L. donovani infection in the liver via mechanisms that are independent of granuloma formation and induction of apoptosis. These results indicate important roles for CD95 and CD95L that are unrelated to regulation of apoptosis in the early control of L. donovani infection.     

Infection of mice with Mycobacterium avium primes CD8+ lymphocytes for apoptosis upon exposure to macrophages

Mycobacterial infection is associated with granuloma formation in which the presence of apoptosis has been recognized. The role of CD4+ T and CD8+ T cells in host protection against mycobacterial infections has been demonstrated. Previous studies, however, have shown that CD8+ T cells have a limited role in host defense against Mycobacterium avium infection, and we hypothesize that M. avium infection could lead to T cell apoptosis. To investigate this hypothesis, C57BL/6 mice were infected with M. avium strain 101, and the rate of apoptosis of splenic lymphocytes cultured ex vivo with peritoneal macrophages was determined and compared with that of controls. When exposed to infected macrophages ex vivo, splenic lymphocytes from M. avium-infected mice underwent apoptosis, as determined by the TUNEL assay. This increased T cell apoptosis above the control level was observed after 3 weeks but not after only 1 week of infection in mice. No splenic T cell apoptosis was observed when lymphocytes from Mycobacterium smegmatis-infected mice were cultured in the presence of M. smegmatis-infected peritoneal macrophages. Likewise, macrophages infected in vitro with heat-killed M. avium did not trigger T cell apoptosis. Culture of macrophages in different chamber from lymphocytes, separated by a transwell membrane, was not associated with increase of apoptosis compared with uninfected control, suggesting a requirement for direct cell-cell interactions to trigger lymphocyte apoptosis. Using a double staining TUNEL followed by anti-mouse CD4 or anti-mouse CD8 monoclonal antibodies, it was observed that only CD8+ T cells but not CD4+ T cells underwent apoptosis at 3 weeks of infection. In conclusion, M. avium infection in C57/BL6 mice for 3 weeks renders CD8+ T cells prone to apoptosis when exposed ex vivo to macrophages infected with M. avium.     

Lack of CD4(+) T cells does not affect induction of CD8(+) T-cell immunity against Encephalitozoon cuniculi infection

Cell-mediated immunity has been reported to play an important role in defense against Encephalitozoon cuniculi infection. Previous studies from our laboratory have underlined the importance of cytotoxic CD8(+) T lymphocytes (CTL) in survival of mice infected with E. cuniculi. In the present study, immune response against E. cuniculi infection in CD4(+) T-cell-deficient mice was evaluated. Similar to resistant wild-type animals, CD4(-/-) mice were able to resolve E. cuniculi infection even at a very high challenge dose (5 x 10(7) spores/mouse). Tissues from infected CD4(-/-) mice did not exhibit higher parasite loads in comparison to the parental wild-type mice. Conversely, at day 21 postinfection, susceptible CD8(-/-) mice had 10(14) times more parasites in the liver compared to control wild-type mice. Induction of the CD8(+) T-cell response in CD4(-/-) mice against E. cuniculi infection was studied. Interestingly, a normal antigen-specific CD8(+) T-cell response to E. cuniculi infection was observed in CD4(-/-) mice (precursor proliferation frequency, 1/2.5 x 10(4) versus 1/10(4) in wild-type controls). Lack of CD4(+) T cells did not alter the magnitude of the antigen-specific CTL response (precursor CTL frequency; 1/1.4 x 10(4) in CD4(-/-) mice versus 1/3 x 10(4) in control mice). Adoptive transfer of immune CD8(+) T cells from both CD4(-/-) and wild-type animals prevented the mortality in CD8(-/-) mice. E. cuniculi infection thus offers an example of an intracellular parasitic infection where CD8(+) T-cell immunity can be induced in the absence of CD4(+) T cells.     

Trypanosoma cruzi-induced immunosuppression: selective triggering of CD4+ T-cell death by the T-cell receptor-CD3 pathway and not by the CD69 or Ly-6 activation pathway.

In a model of experimental Chagas' disease induced with metacyclic forms of Trypanosoma cruzi, CD4+ but not CD8+ T cells undergo T-cell receptor (TCR)-CD3-mediated activation-induced cell death (AICD) in vitro. CD4+ T cells from T. cruzi-infected mice also developed unresponsiveness in proliferative responses to TCR-CD3-mediated stimulation. A linear correlation was found between extent of proliferative unresponsiveness and loss of CD4+ T-cell viability. CD4+ T-cell activation through the CD69 or Ly-6 A/E pathway, on the other hand, did not result in proliferative unresponsiveness compared with controls. Lack of suppression in proliferation assays correlated with lack of AICD by cells stimulated through the CD69 or Ly-6 A/E pathway. Concomitant stimulation through CD69, however, did not rescue CD4+ T cells from CD3-induced death. Flow cytometry study of cells stimulated in vitro showed no defect in interleukin-2 receptor expression by CD4+ T cells from infected donors, which escaped TCR-mediated AICD. In vivo injection of anti-CD3 into acutely infected mice, but not into control mice, led to splenocyte DNA fragmentation and failed to increase splenic CD4+ T-cell numbers. These results show that TCR-CD3-mediated AICD is involved in CD4+ T-cell unresponsiveness in vitro following infection with T. cruzi. In addition, successful activation of these cells through the CD69 and Ly-6 pathways is due to differences in the inability of these stimuli to trigger AICD. Since TCR-CD3-mediated AICD can be induced in vivo in infected mice, these findings may be relevant for the onset of immunological disturbances in the host.     

Toll-like receptor 4 defective mice carrying point or null mutations do not show increased susceptibility to Candida albicans in a model of hematogenously disseminated infection.

We have studied the role of TLR4 in murine defenses against Candida albicans in two TLR4-defective mouse strains: C3H/HeJ mice which have defective TLR4 signaling, and TLR4-/- knockout mice. Both TLR4-defective mice strains experimentally infected with virulent C. albicans cells showed no significant difference in survival as compared with their respective controls. Recruitment of neutrophils to the peritoneal cavity of i.p. infected mice was not affected in TLR4-/-animals, but significantly enhanced in C3H/HeJ mice, compared with their control mice. In vitro production of TNF-alpha by macrophages from both types of TLR4-defective mice, in response to yeasts and hyphae of C. albicans, was not diminished as compared with production by macrophages from wild-type mice. In vitro production of TNF-alpha by yeast-stimulated splenocytes from mice intravenously infected with the low-virulence C. albicans PCA2 strain was not affected in TLR4-defective mice, but the TNF-alpha production in response to hyphae was higher in TLR4-defective than in control animals; the production of IFN-gamma by these splenocytes was similar to controls, as well as the frequency of IFN-gamma-producing CD4+T lymphocytes, indicating that TLR4-defective mice are capable of mounting a Th1 adaptive immune response. Our data indicate that TLR4 is dispensable for murine immune resistance to C. albicans.     

Changes in cell migration‐related molecules expressed by thymic microenvironment during experimental Plasmodium berghei infection: consequences on thymocyte development

We previously showed alterations in the thymus during experimental infection with Plasmodium berghei. Such alterations comprised histological changes, with loss of cortical–medullary limits, and the intrathymic presence of parasites. As the combination of chemokines, adhesion molecules and extracellular matrix (ECM) is critical to appropriate thymocyte development, we analysed the thymic expression of ECM ligands and receptors, as well as chemokines and their respective receptors during the experimental P. berghei infection. Increased expression of ECM components was observed in thymi from infected mice. In contrast, down‐regulated surface expression of fibronectin and laminin receptors was observed in thymocytes from these animals. Moreover, in thymi from infected mice there was increased CXCL12 and CXCR4, and a decreased expression of CCL25 and CCR9. An altered thymocyte migration towards ECM elements and chemokines was seen when the thymi from infected mice were analysed. Evaluation of ex vivo migration patterns of CD4/CD8‐defined thymocyte subpopulations revealed that double‐negative (DN), and CD4⁺ and CD8⁺ single‐positive (SP) cells from P. berghei‐infected mice have higher migratory responses compared with controls. Interestingly, increased numbers of DN and SP subpopulations were found in the spleens of infected mice. Overall, we show that the thymic atrophy observed in P. berghei‐infected mice is accompanied by thymic microenvironmental changes that comprise altered expression of thymocyte migration‐related molecules of the ECM and chemokine protein families, which in turn can alter the thymocyte migration pattern. These thymic disturbances may have consequences for the control of the immune response against this protozoan.     

Decay-accelerating factor (CD55) promotes CD1d expression and Vgamma4+ T-cell activation in coxsackievirus B3-induced myocarditis

BALB/c mice infected with the H3 variant of Coxsackievirus B3 (CVB3) develop severe myocarditis which is initiated by up-regulation of CD1d during infection and CD1d-dependent activation of T cells expressing the Vgamma4 T cell receptor. Previous studies have shown that a mutant variant of the H3 virus which shows reduced binding avidity to one of the known CVB3 virus receptors, decay accelerating factor (DAF), fails to up-regulate CD1d or activate Vgamma4+ cells. To determine if DAF has a role in CD1d expression during infection or Vgamma4+ cell activation, BALB/c and BALB/c DAF-/- mice were infected with CVB3. Infected DAF-/- mice show modest increases in CD1d expression compared to infected wild-type BALB/c mice; and although total numbers of Vgamma4+ cells in the spleen are the same as in BALB/c mice, few Vgamma4+ IFNgamma+ cells are detected in infected DAF-/- animals. Vgamma4+ cell depletion protects infected BALB/c mice from myocarditis but does not protect infected DAF-/- animals, indicating that Vgamma4+ cells are not important to disease in these animals. Anti-CD8 depletion of CD8+ T cells protects infected BALB/c mice but aggravates disease in infected DAF-/- animals, indicating that the immunopathogenicity of viral myocarditis differs in the absence of the DAF virus receptor.     

Host resistance and immune deviation in pigeon cytochrome c T-cell receptor transgenic mice infected with Toxoplasma gondii

Resistance to Toxoplasma gondii has been shown to be mediated by gamma interferon (IFN-gamma) produced by NK, CD4(+), and CD8(+) T cells. While studies of SCID mice have implicated NK cells as the source of the cytokine in acute infection, several lines of evidence suggest that IFN-gamma production by CD4(+) T lymphocytes also plays an important role in controlling early parasite growth. To evaluate whether this function is due to nonspecific as opposed to T-cell receptor (TCR)-dependent stimulation by the parasite, we have examined the resistance to T. gondii infection of pigeon cytochrome c transgenic (PCC-Tg) Rag-2(-/-) mice in which all CD4(+) T lymphocytes are unreactive with the protozoan. When inoculated with the ME49 strain, PCC-Tg animals exhibited only temporary control of acute infection and succumbed by day 17. Intracellular cytokine staining by flow cytometry revealed that, in contrast to infected nontransgenic controls, infected PCC-Tg animals failed to develop IFN-gamma-producing CD4(+) T cells. Moreover, the CD4(+) lymphocytes from these mice showed no evidence of activation as judged by lack of upregulated expression of CD44 or CD69. Nevertheless, when acutely infected transgenic mice were primed by PCC injection, the lymphokine responses measured after in vitro antigen restimulation displayed a strong Th1 bias which was shown to be dependent on endogenous interleukin 12 (IL-12). The above findings argue that, while T. gondii-induced IL-12 cannot trigger IFN-gamma production by CD4(+) T cells in the absence of TCR ligation, the pathogen is able to nonspecifically promote Th1 responses against nonparasite antigens, an effect that may explain the immunostimulatory properties of T. gondii infection.     

Increased susceptibility of Fas ligand‐deficient gld mice to Trypanosoma cruzi infection due to a Th2‐biased host immune response

Infection of BALB / c mice with Trypanosoma cruzi resulted in up‐regulated expression of Fas and Fas ligand (FasL) mRNA by splenic CD4⁺ T cells, activation‐induced CD4⁺ T cell death (AICD), and in Fas : FasL‐mediated cytotoxicity. When CD4⁺ T cells from infected mice were co‐cultured with T. cruzi‐infected macrophages, onset of AICD exacerbated parasite replication. CD4⁺ T cells from T. cruzi‐infected FasL‐deficient BALB gld / gld mice had no detectable AICD in vitro and their activation with anti‐TCR did not exacerbate T. cruzi replication in macrophages. However, infection of BALB gld / gld mice with T. cruzi resulted in higher and more prolonged parasitemia, compared to wild‐type mice. Secretion of Th2 cytokines IL‐10 and IL‐4 by CD4⁺ T cells from infected gld mice was markedly increased, compared to controls. In addition, in vivo injection of anti‐IL‐4 mAb, but not of an isotype control mAb, reduced parasitemia in both gld and wild‐type mice. These results indicate that, besides controlling CD4⁺ T cell AICD and parasite replication in vitro, an intact Fas : FasL pathway also controls the host cytokine response to T. cruzi infection in vivo, being required to prevent an exacerbated Th2‐type immune response to the parasite.     

Different Trypanosoma cruzi strains promote neuromyopathic damage mediated by distinct T lymphocyte subsets

The proliferative response of CD4 and CD8 T lymphocytes obtained from C3H/HeN mice chronically infected with Trypanosoma cruzi strains that differ in virulence, tropism and immunogenicity, was assayed against skeletal muscle, sciatic nerve and spinal cord homogenates. Although both CD4 and CD8 T lymphocytes from mice infected with the RA strain strongly proliferated against the nervous system, no response against skeletal muscle antigens was detected. CD4 and CD8 T lymphocytes from mice infected with the K-98 clone (from CA-I strain) showed low proliferative response against all the antigens assayed. To determine whether the proliferation patterns showed correlation with T cell-mediated neuromuscular damage, passive cell transfer studies were performed. Fifteen days after transfer of CD4 T cells from RA-infected donors (CD4-RA), normal syngeneic recipients displayed exclusively nervous tissue damage, such as perineural, endoneural and/or meningeal inflammatory infiltrates, with predominance of CD4 T cells. Fifteen days after transfer of CD4 T lymphocytes from mice infected with K-98 (CD4-K98), recipients showed inflammatory infiltrates only in skeletal muscle, where CD4 T lymphocytes and macrophages were predominant cells. Recipients of CD8 T cells from RA-infected mice (CD8-RA) showed lesions in both spinal cord and sciatic nerves. Higher percentages of CD8 T cells were observed in comparison with the recipients of CD4-RA or CD4-K98. In contrast, CD8 T cells from K-98-infected donors (CD8-K98) did not induce tissue damage. These results provide evidence that mice infected with T. cruzi populations that differ in their biological characteristics show diverse immune mechanisms that may be involved in the pathogenesis of peripheral nervous system damage.     

CD40 is required for the optimal induction of protective immunity to Mycobacterium avium

C57Bl/6 mice and mice deficient in the CD40 molecule were infected with three strains of Mycobacterium avium. Two of the M. avium strains proliferated more extensively in CD40-deficient (CD40-/-) mice than in control mice. The increased susceptibility to infection of CD40-/- mice was associated with the generation of poorer interleukin-12 (IL-12) p40 and interferon-gamma (IFN-gamma) responses as compared to the controls, suggesting a role for CD40 in the development of protective immunity. In contrast, direct triggering of CD40 on infected macrophages failed to induce any anti-mycobacterial activity in infected macrophages.     

Resistance to Toxoplasma gondii in mice infected as neonates or exposed in utero.

Mice were exposed to the protozoan parasite Toxoplasma gondii in utero or were infected as neonates in order to identify and characterize resistance mechanisms that function protectively during the first weeks after birth. About one-half of the mice born of mothers fed T. gondii cysts at 11 days of gestation survived to weaning age or beyond. No effect of major histocompatibility complex (MHC) haplotype on early survival was observed in a group of backcross progeny; however, long-term survival was strongly dependent on MHC haplotype. The ability of mice infected as neonates to survive until weaning was found to depend on gamma interferon and on Thy-1+ cells but not on CD4+ or CD8+ cells. Mice that survived to maturity after infection as neonates were slightly more resistant to challenge with virulent T. gondii parasites than were sham-infected controls but were less resistant than were mice infected as adults. Together the results indicate the following. (i) Mice congenitally infected with T. gondii have a gamma interferon-dependent mechanism of early resistance that involves Thy-1+ cells but not CD4+ or CD8+ cells. (ii) This mechanism is not under MHC-linked genetic control. (iii) Mice that exhibit long-term survival after congenital infection acquire a modest degree of protection against reinfection with virulent organisms. (iv) The extent of long-term survival of congenitally infected neonates, like that in mice infected as adults, is influenced by MHC genes, presumably via MHC-restricted CD4+ and/or CD8+ cells.     

Interleukin-12 neutralization alters lung inflammation and leukocyte expression of CD80, CD86, and major histocompatibility complex class II in mice infected with Histoplasma capsulatum

Histoplasma capsulatum induces a cell-mediated immune response in lungs and lymphoid organs of mammals. Resolution of primary infection in mice depends on interleukin-12 (IL-12), since neutralization of this monokine increases susceptibility to infection. The present study was designed to determine if blockade of IL-12 disrupts the protective immune response by altering the influx of lineage-specific cells into infected lungs and the numbers of cells expressing CD80, CD86, CD119, and major histocompatibility complex class II (MHC II) molecules. In mice given anti-IL-12, there was a 2.5-fold decrease in total numbers of T cells on days 3 to 10 of infection and a 4-fold increase in Mac-1/Gr-1(+) cells on days 7 and 10 compared to infected controls. CD80(+) lung cells from anti-IL-12-treated mice were 2- to 3-fold greater than those from controls on days 7 and 10, whereas the total numbers of CD86(+) cells were 2- to 3-fold less and MHC II(+) cells were 1.5- to 2-fold less on days 3 and 5. Cells expressing CD119 were reduced 1.5-fold on day 5. Treatment with monoclonal antibodies (MAb) to CD80, CD86, or both reduced the fungal burden slightly compared to that in rat immunoglobulin G-treated controls, whereas after IL-12 neutralization, blocking of CD80 reduced the tissue burden by 2. 5-fold and this correlated with a decrease in IL-4. Regardless, mortality was not altered by treatment with MAb to CD80 or CD86. We conclude that (i) IL-12 neutralization alters the nature of the inflammatory response in lungs and the expression of CD80 and CD86 on lineage-specific cells, (ii) the immune response during infection with H. capsulatum is controlled via mechanisms independent of the CD80 and CD86 costimulatory pathways, and (iii) decreased expression of CD86 and MHC II may modulate generation of optimal protective immunity.     

The fate of heterologous CD4+ T cells during Leishmania donovani infection

Little is currently understood about the consequences of chronic parasitic infection for the fate of memory CD4+ T cells that recognize heterologous antigens, e.g. resulting from prior infections or vaccination. Here, we address how Leishmania donovani infection affected the fate of non-cross-reactive (OVA)-specific memory CD4+ T cells. DO11 cells were adoptively transferred into naive recipient mice, which were then immunized to generate memory DO11 cells. After 6 weeks, mice were infected with L. donovani and the fate of DO11 cells was determined. L. donovani infection stimulated an approximately threefold expansion in the total number of CD4+ T cells and DO11 cells, compared to that observed in uninfected mice. DO11 T cells were more actively dividing in infected mice, as judged by 5-bromo-2' deoxyuridine labeling, whereas their rate of apoptosis in control and infected mice was identical. Both CD45RBhiCD44lo naive T cells and to a greater extent CD45RBloCD44hi memory DO11 cells increased in number in the spleens of infected mice, whereas no changes occurred to DO11 cell number or phenotype in the draining lymph nodes. These data indicate that heterologous CD4+ T cells may actively divide during chronic infectious diseases, with important implications for how chronic infection may impact on heterologous immunity.     

Role of CD8+ and CD4+ T lymphocytes in jejunal mucosal injury during murine giardiasis

T-cell-mediated pathogenesis has been documented in various idiopathic and microbially induced intestinal disorders. Diffuse microvillous shortening seen in giardiasis is responsible for disaccharidase insufficiencies and malabsorption of electrolytes, nutrients, and water. Other mucosal changes include crypt hyperplasia and increased numbers of intraepithelial lymphocytes (IEL). A recent report using an athymic mouse model of infection showed that these epithelial injuries were dependent on T cells. The aim of the present study was to identify which subset of superior mesenteric lymph node (SMLN) T cells were responsible for mucosal alterations in giardiasis. CD4+ and CD8+ T cells, as well as whole lymphocyte populations, were isolated from SMLN of Giardia muris-infected mice for adoptive transfer. Jejunal segments of recipient mice were assessed for brush border ultrastructure, sucrase activity, crypt/villus ratio, and IEL numbers. Mice that received enriched CD8+ and whole SMLN lymphocytes, but not CD4+ T cells, from infected donors showed diffuse shortening of microvilli, loss of brush border surface area, impaired sucrase activity, and increased crypt/villus ratios compared to respective controls. Transfer of whole SMLN lymphocytes, as well as enriched CD4+ or CD8+ T cells, from infected donors led to increased IEL numbers in the recipient jejunum. The findings indicate that loss of intestinal brush border surface area, reduced disaccharidase activities, and increased crypt/villus ratios in giardiasis are mediated by CD8+ T cells, whereas both CD8+ and CD4+ SMLN T cells regulate the influx of IEL.     

Expression of class II,but not class I,major histocompatibility complex molecules is required for granuloma formation in infection with Schistosoma mansoni

Previous studies have suggested that granulomatous inflammation in schistosomiasis is mediated by CD4⁺ T helper lymphocytes sensitized to parasite egg antigens. However, CD8⁺ T cells have also frequently been associated with the immune response to schistosome eggs. To examine more precisely the role of CD4⁺ and CD8⁺ T cells in the pathology of the schistosomal infection, we used mice with targeted mutations in major histocompatibility complex (MHC) class II or class I molecules. These mutations lead, respectively, to the virtual absence of CD4⁺ and CD8⁺ T cells. The results clearly show that schistosome-infected MHC class II mutant mice failed to form granulomas around parasite eggs. In contrast, infected MHC class I mutant mice displayed characteristic granulomatous lesions that were comparable to those in wild-type control mice. Moreover, lymphoid cells from MHC class II mutant mice were unable to react to egg antigens with either proliferative or cytokine [interferon-gamma, interleukin (IL)-4, IL-10] responses; nor were they able to present egg antigens to specifically sensitized CD4⁺ T helper cells from infected syngeneic control mice. By comparison, cells from MHC class I mutant mice exercised all these functions in a manner comparable with those from wild-type controls. These observations clearly demonstrate that schistosomal egg granulomas are mediated by MHC class II-restricted CD4⁺ T helper cells. They also suggest that CD8⁺ T cells do not become sensitized to egg antigens and play little role, if any, in the pathogenesis of schistosomiasis.