Endogenous interleukin-12 is involved in resistance of mice to Mycobacterium avium complex infection.

Acquired cellular resistance against Mycobacterium avium complex (MAC) infections involves the induction of Th1 type gamma interferon (IFN-gamma)-producing T cells. Interleukin-12 (IL-12) is a cytokine involved in the control of IFN-gamma production by T cells and NK cells. The role of IL-12 in the response to MAC infection was investigated. Depletion of endogenous IL-12 by injection of monoclonal antibody prior to and during intranasal infection with MAC resulted in an 150- to 550-fold increase in bacterial load in lung, spleen, and liver homogenates by 10 weeks postinfection. Depletion of IL-12 abrogated the ability of spleen cell cultures to produce IFN-gamma in response to stimulus with live MAC. IL-12-depleted mice showed a 75% decrease in the number of inflammatory cells entering the lungs following intranasal infection with MAC, with significant reductions in cytotoxic activity and nitric oxide production by lung cells. This work suggests that IL-12 plays a major role in the activation of IFN-gamma-producing cells during MAC infection.     

Tumor necrosis factor alpha and interleukin-12 contribute to resistance to the intracellular bacterium Brucella abortus by different mechanisms.

Both interleukin-12 (IL-12) and tumor necrosis factor alpha (TNF-alpha) are produced early in intracellular bacterial infection. Depletion of either IL-12 or TNF-alpha by a single injection of specific antibody 4 h before the injection of Brucella abortus 19 led to the exacerbation of infection 2 weeks later. Whereas the effect of IL-12 depletion on resistance was persistent and exacerbation was still significant 6 weeks later, the bacterial numbers in mice depleted of TNF-alpha were similar to the bacterial numbers in control infected mice by 6 weeks postinfection. Massive splenomegaly, which is often seen in 2-week Brucella-infected mice, was not observed in IL-12- or TNF-alpha-depleted mice. Both IL-12- and TNF-alpha-depleted mice showed reduced cell accumulation in the spleen compared with the massive cell accumulation in control infected mice. Granuloma formation in livers was much reduced in IL-12-depleted mice but not in TNF-alpha-depleted mice. Gamma interferon (IFN-gamma) production by cells from TNF-alpha-depleted mice was not significantly different from that of cells from control infected mice. In contrast, the production of IFN-gamma by both CD4+ and CD8+ T cells from IL-12-depleted mice was greatly reduced, compared with that from control infected mice. This effect was still observed when the antibody injection was delayed for up to 7 days postinfection, but injections of anti-IL-12 antibody into mice with established Brucella infection had no significant effect on IFN-gamma production by T cells. Taken together, these results suggested that IL-12 contributed to resistance mainly via an IFN-gamma-dependent pathway and had a profound effect on the induction of acquired cellular resistance. In contrast, TNF-alpha was involved in resistance possibly via direct action on effector cells and may not be essential for the induction of acquired cellular resistance.     

Gamma interferon-dependent temporary resistance to acute Toxoplasma gondii infection independent of CD4+ or CD8+ lymphocytes.

It is well established that resistance to acute primary Toxoplasma gondii infection is mediated by a gamma interferon (IFN-gamma)-dependent mechanism. The present in vivo experiments were undertaken to investigate the cellular basis for this resistance. We show here that immunocompetent T. gondii-infected C57BL/6 (B6) mice treated with anti-IFN-gamma or with anti-Thy-1 or anti-asialo-GM1 antibodies die sooner than infected mice treated with antibodies that deplete both CD4+ and CD8+ T lymphocytes. Thy-1+ CD4- CD8- cells accumulated in the peritoneal cavities of B6 mice during the early stages of an intraperitoneal infection but did not accumulate in sham-infected control mice, and substantial numbers of Thy-1+ CD4- CD8- cells were recovered from the peritoneal cavities of infected B6 mice treated with antibodies that depleted CD4+ and CD8+ lymphocytes. Depletion of Thy-1+ cells reduced IFN-gamma to undetectable levels, whereas depletion of CD4+ and CD8+ cells did not reduce IFN-gamma levels. Thus T. gondii infection in immunocompetent B6 mice elicits Thy-1+ CD4- CD8- cells which either produce protective IFN-gamma themselves or control its production by other cells. It is likely that the function of these Thy-1+ CD4- CD8- cells is to control T. gondii tachyzoites during the early stages of primary infection before specific CD4(+)- and/or CD8(+)-dependent immunity develops.     

Relative importance of CD4+ and CD8+ T cells in the resolution of Chlamydophila abortus primary infection in mice

The role of the specific cellular immune response is well established in Chlamydiaceae infections, but the importance of each T-cell subset seems to be species-dependent. This study was designed to clarify the role of T-cell subsets in the response to Chlamydophila abortus primary infection. C57BL/6 mice were depleted of CD4+ or CD8+, or both, by monoclonal antibody injections and subsequently infected with C. abortus. Mice were killed at intervals and samples were collected for bacteriological and histopathological analysis. Also carried out were spleen cell culture, cytokine quantification, immunolabelling for C. abortus antigen, and a TUNEL assay for apoptosis. CD8+ T cell-depleted mice all died within 12 days of C. abortus infection, while no mortality was observed in the other groups; surprisingly, CD4+ T cell-depleted mice showed lower morbidity (expressed as weight loss) than did a non-depleted (control) group. CD8+ T cell-depleted mice also differed from the other groups in showing a significantly higher chlamydial burden in the liver. CD8+ T cell-depleted mice also had a higher number of apoptotic cells in hepatic inflammatory foci and showed exacerbated IFN-gamma production by spleen cells after specific stimulation. Simultaneous depletion of both T-cell subpopulations led to a chronic infection, but not to early mortality. It is concluded that CD8+ T cells may play a role in the regulatory control of the CD4+ T-cell response and may have a direct cytotoxic or IFN-gamma-mediated effect on infected cells.     

Roles of CD4+ and CD8+ T cells in adjuvant activity of diesel exhaust particles in mice

Through an imbalance in Th1 and Th2 cytokine profiles, diesel exhaust particles (DEP) are thought to induce Th2-dominated IgE and IgG1 production. However, the roles of CD4+ and CD8+ T-cell subtypes in the increased immune responses to antigen in mice exposed to DEP are unclear. In the present study, we investigated whether treatment with anti-CD4 or anti-CD8 mAb abrogated the adjuvant activity of DEP. On day -1 and day 1, each group of mice was injected intraperitoneally with anti-CD4, anti-CD8, or rat IgG (vehicle). On day 0, the mice were immunized with ovalbumin (OVA) or OVA plus DEP. After 3 weeks, each mouse was boosted with 10 microg of OVA alone. On day 7 after the first injection with OVA+DEP or OVA alone, the numbers of total, IA+, CD80+/IA+ and CD86+/IA+ cells in peritoneal exudate cells (PEC) were higher in OVA+DEP-immunized mice than in OVA-immunized mice. Depletion of CD8+ cells resulted in a modulation of the production of granulocyte-macrophage colony-stimulating factor, IL-12 and PGE(2) in peritoneal exudate fluid from OVA+DEP-immunized mice. On day 28, DEP injection markedly increased IL-4 production in the culture supernatants of spleen cells from CD4+ or CD8+-depleted mice. Depletion of CD8+ cells in OVA+DEP-immunized mice resulted in a decrease in IFN-gamma production compared with that in OVA-immunized mice. Adjuvant activity of DEP was observed in anti-OVA IgE, anti-OVA IgG1, anti-OVA IgG3, and total IgE production. Depletion of CD4+ T cells abrogated the adjuvant effect of DEP on anti-OVA IgE, and anti-OVA IgG1 production in plasma. However, depletion of CD8+ T cell inhibited the upregulated anti-OVA IgG3 production. These findings suggest that DEP injection may affect not only the function of CD4+ cells but also that of CD8+ T-cell subsets to modulate the synthesis of proinflammatory cytokine in PEC and type-1 and type-2 cytokine production in spleens.     

Resistance to Cryptococcus neoformans infection in the absence of CD4+ T cells.

Previous studies of Cryptococcus neoformans infection have revealed a role for CD4+ T cells and CD8+ T cells in anticryptococcal resistance in the lungs, but such a role has been revealed only for CD4+ T cells in the brains of experimentally infected mice. In this study, we found that mice genetically engineered to lack CD4+ T cells could be successfully vaccinated to express resistance to a rechallenge with Cryptococcus neoformans, provided the challenge dose was kept to lower than 1000 organisms per mouse. The challenge infection was uniformly lethal for unvaccinated control mice. Depletion of CD8+ T cells weakened this resistance to re-challenge: both na?ve and vaccinated mice that were treated with antibody raised against CD8+ T cells died significantly earlier than did mice that received an irrelevant control antibody. In vitro, purified CD8+ T cells taken from draining lymph nodes of antigen-experienced mice were less efficient than were identically prepared CD4+ T cells at stimulating the cells of a transformed microglial cell line to inhibit C. neoformans proliferation, possibly mirroring the inferiority of CD8+ T-cell-mediated protection observed in vivo. RNase protection assays showed similar IFN-gamma mRNA levels in both lymphocyte subsets. Class II major histocompatibility antigen expression was up-regulated strikingly on microglia cultured with IFN-gamma, but class I expression was less dramatically affected. Therefore microglial cell interaction may be more greatly enhanced with CD4+ cells than with CD8+ cells.     

CD8+ T cells are the major lymphocyte subpopulation involved in the protective immune response to Toxoplasma gondii in mice.

The ability of the major T cell subsets to adoptively transfer resistance to T. gondii infection was studied. Spleen cells harvested from mice with a 3-month T. gondii infection and cells from uninfected mice were enriched for T cells by nylon/wool purification. Adoptive transfer of these cells from both groups of donor mice led to a significant increase in the survival of syngeneic recipient mice infected intraperitoneally with 20 T. gondii cysts. Increased survival was mediated particularly by CD4-depleted but also, to a lesser extent, CD8-depleted subpopulations. These results were confirmed in T cell reconstituted athymic nude mice. Unfractionated T cells from chronically infected donors produced a significant inhibition of cyst formation in the brains of recipient mice measured 10 weeks after infection compared with control mice. The inhibition of cyst formation was ablated by pretreating T cells with anti-CD8 antibody and complement, but not anti-CD4 antibody and complement. Mice receiving cells from infected donors produced an early increase in their IgG1 and IgG2a antibody titres compared with mice given cells from uninfected animals. The depletion of either CD8+ or CD4+ immune cells appeared to have little effect on the antibody responses in recipient mice and there was no correlation between antibody levels and immunity. The results indicate that CD8+ T lymphocytes from convalescent T. gondii-infected BALB/c mice are the principal mediators of resistance to T. gondii, although CD4+ T cells appear to be involved during the acute phase of infection.     

Role of gamma interferon and tumor necrosis factor alpha in resistance to Salmonella typhimurium infection.

In mice infected with a sublethal dose of Salmonella typhimurium, the injection of an anti-gamma interferon (IFN-gamma) monoclonal antibody increased bacterial proliferation in the spleen and led to death on day 7 or 8. Depletion of both CD4+ and CD8+ T cells with monoclonal antibodies in vivo had a much less marked effect during the first week of infection than the administration of anti-IFN-gamma antibodies, suggesting that cells other than T lymphocytes participate in the production of IFN-gamma at this time. Administration of anti-tumor necrosis factor alpha (TNF-alpha) antibodies to mice infected with a sublethal dose of S. typhimurium induced the same effect as anti-IFN-gamma antibodies, while the administration of both antibodies resulted in a synergistic interaction. When mice were infected with an avirulent strain of S. typhimurium and challenged on day 7 either with a virulent strain of S. typhimurium or with Listeria monocytogenes, their resistance to reinfection was slightly depressed by anti-IFN-gamma or anti-TNF-alpha antibodies given 1 day before challenge and much more strongly depressed by the simultaneous administration of both antibodies. Taken together, these results indicate that IFN-gamma and TNF-alpha play an essential role in acquired resistance during the early phase of S. typhimurium infection.     

Do L3T4+ T cells act as effector cells in protection against influenza virus infection

This study aimed to analyse the roles of Lyt 2+ and L3T4+ memory T-cell subpopulations in murine influenza infection. Previous work has shown that Lyt 2+ cytotoxic T-cell (Tc) clones can adoptively transfer protection. We therefore wished to see whether L3T4+ (Th) cells could also act as protective effector cells. Donors for adoptive cell transfer were thymectomized mice, depleted in vivo of either Lyt 2+ or L3T4+ T cells with monoclonal antibodies (MAb) and then infected with influenza virus (A/X31). Primed spleen cells, after removal of the B cells, were transferred into irradiated hosts infected simultaneously or persistently with a heterologous influenza virus and the effect on lung virus replication determined. Depletion of L3T4+ T cells suppressed the formation of IgG antibodies after influenza virus infection, indicating significant depletion of T-helper function. Yet Lyt 2+ class I MHC-restricted Tc cells were effectively primed in these mice, albeit to half the normal level. Adoptive transfer of the Lyt 2+ memory T cells cleared virus in a persistent infection within 6 days. Spleen cells selected for L3T4+ T cells cleared virus within 21 days of transfer in a simultaneous infection and reduced viral titres in a persistent infection, but not as effectively as L3T4+-depleted spleen cells. Although no Lyt 2+ cells were detected by fluorescence staining in Lyt 2+-depleted spleens, we could detect low levels of class I MHC-restricted influenza-specific Tc memory cells in host spleens following influenza infection. Therefore, whether the early viral clearance is solely due to L3T4+ T cells is not clear. Lyt 2+ memory T cells appear more efficient in this respect than L3T4+ memory T cells.     

Role of gamma interferon and tumor necrosis factor alpha during T-cell-independent and -dependent phases of Mycobacterium avium infection.

To design an effective immunotherapy for Mycobacterium avium infections, the protective host response to the infection must be known. Here we analyzed the role of gamma interferon (IFN-gamma) and tumor necrosis factor alpha (TNF-alpha) in the innate and acquired responses to M. avium infections in mice. T-cell depletion studies showed that CD4+ T cells were required for control of the infection. CD(4+)-depleted mice showed enhanced bacterial proliferation and at the same time showed a reduction in the level of expression of both IFN-gamma and TNF-alpha mRNAs in spleen cells. In contrast, M. bovis BCG immunization restricted M. avium proliferation and at the same time promoted expression of the mRNAs for the two cytokines. In vivo depletion studies using specific monoclonal antibodies showed that both IFN-gamma and TNF-alpha are involved in an early protection possibly involving NK cells, and furthermore, IFN-gamma is involved in the later T-cell-protective response to infection. In vivo neutralization of IFN-gamma during M. avium infection also blocked the priming for enhanced TNF-alpha secretion triggered by endotoxin. Both cytokines were found to be involved in the resistance expressed in BCG-immunized animals and exhibited additive bacteriostatic effects in vitro on bone marrow-derived macrophages infected with different strains of M. avium. These data suggest that both cytokines act in an additive or synergistic fashion in the induction of bacteriostasis and that IFN-gamma is also involved in priming TNF-alpha secretion.     

In vivo acute depletion of CD8(+) T cells before murine cytomegalovirus infection upregulated innate antiviral activity of natural killer cells

In this study, we investigated the effect of depletion of CD8(+) T cells on the activity of natural killer (NK) cells at an early phase of murine cytomegalovirus (MCMV) infection. For CD8(+) T cell depletion, mice were intraperitoneally treated with anti-CD8 mAb, purified from 2.43 hybridoma, for 2 consecutive days before or after infection. Three days after infection, we found that an acute depletion of CD8(+) T cells before infection caused a significant decrease in the viral load in liver and spleen. This effect coincided with an increase in numbers of CD3(-) NK1.1(+) cells in spleen and their expression of the early activation molecule CD69. Although cytolytic activity of NK cells increased on day 3 of infection in CD8-depleted mice, the level of IFN-gamma decreased in serum and supernatant of cultured spleen cells. In contrast to the effect of acute depletion of CD8(+) T cells before infection, the depletion after infection had no effect on the viral load or number and cytolytic function of NK cells. Lack of effects of CD8(+) T cell depletion on the viral load and NK cytolytic activity is also observed in CD8(+) knockout mice. In conclusion, the results suggest that an acute depletion of CD8(+) T cells before MCMV infection effectively upregulated the antiviral activity of NK cells. This effect appears to be mediated through an increase in numbers, activation and cytolytic activity of NK cells.     

38 000 MW antigen-specific major histocompatibility complex class I restricted interferon-gamma-secreting CD8+ T cells in healthy contacts of tuberculosis.

CD8+ T lymphocytes are required to protect mice against Mycobacterium tuberculosis, although in early infection the mechanism appears not to be via perforin or granzyme-mediated lysis of the infected target, and may be via interferon-gamma (IFN-gamma) production. We therefore investigated whether CD8+ T cells specific for the immunoprotective 38 000 MW antigen of M. tuberculosis could be detected in infected humans. Using a recombinant vaccinia virus expressing the 38 000 MW antigen of M. tuberculosis (rV38) and a control vaccinia virus (rVras) we demonstrated that both viruses stimulated IFN-gamma production from freshly isolated peripheral blood mononuclear cells (PBMC) in a 36-hr enzyme-linked immunospot assay. Cell depletion and antibody blockade established that the bulk of the 38 000 MW antigen-specific IFN-gamma response was mediated by CD8+, major histocompatibility complex class I-restricted T cells, whereas the anti-vaccinia virus response was predominantly mediated by CD4+ T cells. In further evaluations PBMC from all seven healthy tuberculosis-exposed contacts had a 38 000 MW antigen-specific IFN-gamma response, whereas seven patients with untreated sputum-positive pulmonary tuberculosis had very low levels of 38 000 antigen-specific IFN-gamma-producing cells. These preliminary observations demonstrate the utility of recombinant vaccinia viruses in restimulating freshly isolated CD4+ and CD8+ T cells. The bias towards a higher frequency of IFN-gamma-producing CD8+ T cells in contacts rather than patients may indicate a protective role for CD8+ cells in human tuberculosis.     

Effect of spleen cell populations on resolution of Cryptosporidium parvum infection in SCID mice.

Persistent infection was established in SCID mice given 10(7) Cryptosporidium parvum oocysts. Nine groups of infected SCID mice were inoculated with 10(6), 10(5), or 10(4) total spleen cells, CD8-depleted spleen cells, or CD4-depleted spleen cells from naive BALB/c donors. Infection was significantly reduced in all treatment groups. The most profound effect occurred with spleen cell preparations containing CD4 T lymphocytes but depleted of CD8 T lymphocytes.     

T-cell recognition of a cross-reactive antigen(s) in erythrocyte stages of Plasmodium falciparum and Plasmodium yoelii: inhibition of parasitemia by this antigen(s).

In the current study, we investigated the presence of a cross-reactive antigen(s) in the erythrocyte stage from Plasmodium yoelii (265 BY strain) and Plasmodium falciparum through recognition by T cells primed in vivo with antigens from each of these parasites. BALB/c mice are naturally resistant to P. falciparum but are susceptible to P. yoelii infection. Mice that had recovered from P. yoelii primary infection became resistant to a second infection. A higher in vitro proliferative response to a soluble blood stage preparation of P. falciparum was observed in splenic cells from immune animals than in those from mice with a patent P. yoelii infection. The antigen-induced proliferative response was enhanced when animals were exposed to a secondary infection. Animals exposed to a challenge infection were treated with anti-CD4 or anti-CD8 monoclonal antibodies to deplete the corresponding subset of T cells. There was a marked diminution in P. falciparum antigen-induced proliferative response in the total splenic cell populations from CD8-depleted but not from CD4-depleted mice. In CD8-depleted and nondepleted animals, the antigen-induced proliferation in the total cell populations was markedly lower than in the T-cell-rich populations, indicating inhibitory activities of B cells and/or macrophages. There was no such difference in the stimulation between total and T-enriched cell populations from CD4-depleted animals. Flow cytometry analysis demonstrated the presence of an almost equal percentage of CD8+ (59.6%) and CD4+ (64%) T cells in the spleen preparations following in vivo depletion of CD4- and CD8-bearing T cells, respectively. When cultured with P. yoelii blood stage antigen, splenocytes from animals immunized with P. falciparum antigen displayed a significant proliferative response which was markedly diminished by treatment with anti-Thy-1.2 antibody plus complement. Animals immunized with P. falciparum antigen and then challenged with P. yoelii blood stage parasites displayed about a 50% lower level of parasitemia. These results demonstrated the existence of a cross-reactive antigen(s) between a murine and a human Plasmodium species, as determined from both in vivo and in vitro biological assays, and indicated the reactivity of mainly CD8+ T cells with this antigen.     

Organ- and disease-stage-specific regulation of Toxoplasma gondii-specific CD8-T-cell responses by CD4 T cells

Toxoplasma gondii induces a persistent central nervous system infection, which may be lethally reactivated in AIDS patients with low CD4 T-cell numbers. To analyze the role of CD4 T cells for the regulation of parasite-specific CD8 T cells, mice were infected with transgenic T. gondii expressing the CD8 T-cell antigen beta-galactosidase (beta-Gal). Depletion of CD4 T cells prior to infection did not affect frequencies of beta-Gal(876-884)-specific (consisting of residues 876 to 884 of beta-Gal) CD8 T cells but resulted in a pronounced reduction of intracerebral beta-Gal-specific gamma interferon (IFN-gamma)-producing and cytolytic CD8 T cells. After cessation of anti-CD4 treatment a normal T. gondii-specific CD4 T-cell response developed, but IFN-gamma production of intracerebral beta-Gal-specific CD8 T cells remained impaired. The important supportive role of CD4 T cells for the optimal functional activity of intracerebral CD8 T cells was also observed in mice that had been depleted of CD4 T cells during chronic toxoplasmosis. Reinfection of chronically infected mice that had been depleted of CD4 T cells during either the acute or chronic stage of infection resulted in an enhanced proliferation of beta-Gal-specific IFN-gamma-producing splenic CD8 T cells. However, reinfection of chronically infected mice that had been depleted of CD4 T cells in the acute stage of infection did not reverse the impaired IFN-gamma production of intracerebral CD8 T cells. Collectively, these findings illustrate that CD4 T cells are not required for the induction and maintenance of parasite-specific CD8 T cells but, depending on the stage of infection, the infected organ and parasite challenge infection regulate the functional activity of intracerebral CD8 T cells.     

Depletion of T-cell subpopulations results in exacerbation of myocarditis and parasitism in experimental Chagas' disease.

The contribution of T-cell subpopulations to immunopathology in murine Trypanosoma cruzi infection was studied by using in situ localization of lymphocytes and in vivo depletion of T-lymphocyte populations. CD8+ T cells were the major lymphocyte population in the inflamed hearts of C3H/HeSnJ mice infected with the Sylvio X10/4 clone of T. cruzi at all time points of the acute and chronic phases of the infection examined. Depletion of CD8+ and/or CD4+ T cells beginning on day 20 of the infection resulted in a moderate decrease in the inflammation and an increase in parasite burden in the hearts of mice at day 30 of infection. Longer-term depletion, beginning at day 20 and extending as long as 200 days of infection, resulted in an increased inflammatory response in the heart. A large proportion of the inflammatory cells in the hearts of anti-CD8- or anti-CD4- and anti-CD8-treated mice were Thy1+ and CD4- CD8-. At 200 days of infection, the increased inflammation was accompanied by an increase in the parasite load in the heart. These results show that T-cell subset depletion does not prevent the inflammatory response associated with acute and chronic T. cruzi infection. The increased parasite load in T-cell-depleted mice also demonstrates the participation of these T-cell subsets in regulation of parasite load throughout the course of the infection. The increased inflammatory response despite T-cell depletion and in association with increased numbers of tissue parasites suggests that intracellular parasites are a driving force behind the inflammatory response in chronic murine T. cruzi infection.     

A T cell-independent protective host response against Cryptococcus neoformans expressed at the primary site of infection in the lung.

T cell-independent host resistance expressed against a primary lung infection with Cryptococcus neoformans was investigated. Following intratracheal inoculation of the yeast, BALB/cBy scid/scid mice or CD4+ plus CD8+ T cell-depleted BALB/cBy mice developed a primary lung infection that remained stable for several weeks before progressing and disseminating to kill the host. By contrast, normal BALB/cBy hosts resolved the infection after 4 to 8 weeks. Thy+ CD4- CD8- cells were found to accumulate in the pulmonary alveoli of infected scid/scid or normal mice. Depletion of these cells caused the infection to progress more rapidly and resulted 4 weeks later in a 30- to 70-fold increase in yeast numbers in the lungs and dissemination to extrapulmonary sites. Cytofluorometric studies revealed that the Thy+ CD4- CD8- cells responsible were negative for the CD3 T cell marker. A small percentage of these Thy+ CD3- cells expressed asialo-Gm1, but treatment with asialo-Gm1 antibody did not have the same infection-enhancing effect as Thy-1 monoclonal antibody treatment. Further experiments revealed that Thy-1 monoclonal antibody treatment had no effect on the establishment of infectious foci in the brain or liver following intravenous inoculation of the yeast. The data point to the existence of an early resistance mechanism for which Thy+ CD3- CD4- CD8- cells are essential. This mechanism of host defense, while insufficient for complete protection, may be capable of delaying the development of cryptococcal meningoencephalitis by restricting the growth of the yeast at primary sites of infection in the lungs, even in immunodeficient mice.     

Differential activation of Brucella-reactive CD4+ T cells by Brucella infection or immunization with antigenic extracts.

In order to induce acquired cellular resistance to facultative bacterial pathogens, infection with live organisms is required. We have previously demonstrated that spleen cells from Brucella-infected mice produced gamma interferon (IFN-gamma) and interleukin-2 (IL-2) in response to Brucella antigens in vitro, while spleen cells from mice immunized with soluble Brucella proteins (SBP) produced substantial amounts of IL-2 but no detectable amount of IFN-gamma. In this study, we further analyzed the response of T cells from Brucella-infected mice and SBP-immunized mice and demonstrated that CD4(+)-enriched cells from SBP-immunized mice also produced significant amounts of IL-4, which was not detected in bulk cultures of spleen cells from infected mice. Limiting dilution analysis showed that infection resulted in a higher precursor frequency of IFN-gamma-producing CD4+ T cells and a lower precursor frequency of IL-4-producing CD4+ T cells, while immunization with SBP resulted in a higher precursor frequency of IL-4-producing cells and a very low frequency of IFN-gamma-producing cells. The precursor frequencies of IL-2-producing cells for the two groups were similar. Furthermore, IFN-gamma-producing CD4+ T cells from infected donor mice were capable of mediating resistance against challenge infection in recipient mice, but IL-4-producing CD4+ T cells from immunized mice failed to do so. These results indicate that the form of antigen has a profound influence on the outcome of the immune response. The results are discussed in light of the supposed dichotomy between Th1 and Th2 cytokine responses.     

T Cells Augment Monocyte and Neutrophil Function in Host Resistance against Oropharyngeal Candidiasis

The purpose of this study was to identify the cell populations involved in recovery from oral infections with Candida albicans. Monoclonal antibodies specific for CD4+ cells, CD8+ cells, and polymorphonuclear leukocytes were used to deplete BALB/c and CBA/CaH mice of the relevant cell populations in systemic circulation. Monocytes were inactivated with the cytotoxic chemical carrageenan. Mice were infected with 10(8) C. albicans yeast cells and monitored for 21 days. Systemic depletion of CD4+ and CD8+ T lymphocytes alone did not increase the severity of oral infection compared to that of controls. Oral colonization persisted in animals treated with head and neck irradiation and depleted of CD4+ T cells, whereas infections in animals that received head and neck irradiation alone or irradiation and anti-CD8 antibody cleared the infection in a comparable fashion. The depletion of polymorphonuclear cells and the cytotoxic inactivation of mononuclear phagocytes significantly increased the severity of oral infection in both BALB/c and CBA/CaH mice. High levels of interleukin 12 (IL-12) and gamma interferon (IFN-gamma) were produced by lymphocytes from the draining lymph nodes of recovering animals, whereas IL-6, tumor necrosis factor alpha, and IFN-gamma were detected in the oral mucosae of both na?ve and infected mice. The results indicate that recovery from oropharyngeal candidiasis in this model is dependent on CD4+-T-cell augmentation of monocyte and neutrophil functions exerted by Th1-type cytokines such as IL-12 and IFN-gamma.