T-cell subsets and T-cell antigens in protective immunity against experimental leishmaniasis

Immunoregulatory mechanisms and CD4-CD8- (double negative) T cell subpopulations in human cutaneous leishmaniasis: a balancing act between protection and pathology

Cellular immune responses directed against protozoan parasites are key for controlling pathogen replication and disease resolution. However, an uncontrolled, or improperly controlled, response can be deleterious to the host in terms of both allowing for the establishment of pathology, as well as less effective establishment of memory responses. Human cutaneous leishmaniasis is a disease caused by the infection with Leishmania spp. following a bite from the sandfly, the natural vector of this disease. Tens of millions worldwide are currently infected with Leishmania and no effective vaccines have been developed to date. In the face of the complexity presented by the interaction between a host (humans) with the parasite, Leishmania, and the fact that this parasite is inoculated by another complex, biologically active, vector, the sandfly, it is clearly important to study the immunoregulatory mechanisms that are induced in humans naturally infected by this parasite if we hope to develop effective vaccines and immunotherapeutic treatments in the future. Our laboratory has focused over the years on the study of the local and systemic T cell response during the first episode of cutaneous leishmaniasis suffered by individuals before they undergo antimony treatment. The goal of this review is to briefly outline our findings with hopes of putting our most recent studies concerning the dichotomy between alpha/beta TCR and gamma/delta TCR expressing, CD4-CD8- (double negative-DN) T cells in the context of a balanced immune response against Leishmania and to discuss the implications of these findings toward our understanding of human leishmaniasis.

Flow cytometric determination of cellular sources and frequencies of key cytokine-producing lymphocytes directed against recombinant LACK and soluble Leishmania antigen in human cutaneous leishmaniasis

Leishmaniasis, caused by infection with the protozoan parasite Leishmania, affects millions of individuals worldwide, causing serious morbidity and mortality. This study directly determined the frequency of cells producing key immunoregulatory cytokines in response to the recombinant antigen Leishmania homolog of receptors for activated kinase C (LACK) and soluble leishmania antigen (SLA), and it determined relative contributions of these antigens to the overall cytokine profile in individuals infected for the first time with Leishmania braziliensis. All individuals presented with the cutaneous clinical form of leishmaniasis and were analyzed for proliferative responses to LACK antigen and SLA, frequency of lymphocyte subpopulations (analyzed ex vivo), and antigen-induced (LACK and SLA) cytokine production at the single-cell level (determined by flow cytometry). The following were determined. (i) The Th1-type response previously seen in patients with cutaneous leishmaniasis is due to gamma interferon (IFN-gamma) production by several different sources, listed in order of contribution: CD4(+) T lymphocytes, CD4(-), CD8(-) lymphocytes, and CD8(+) T lymphocytes. (ii) SLA induced a higher frequency of lymphocytes producing IFN-gamma and tumor necrosis factor alpha (TNF-alpha) than did LACK. (iii) LACK induced an activation of monocyte populations as reflected by an increased percentage of CD14-positive cells. (iv) Neither SLA nor LACK induced detectable frequencies of cells producing interleukin-4 (IL-4) or IL-5. These data demonstrated a multifaceted immune response to SLA in human leishmaniasis involving Th1 CD4(+) T lymphocytes (IFN-gamma(+) and IL-10(-)/IL-4(-)), Tc1 CD8(+) T cells (IFN-gamma(+), and IL-10(-)/IL-4(-)), and a high frequency of TNF-alpha-producing lymphocytes. Moreover, it was determined that the recombinant antigen LACK acts as a weak inducer of Th1-type lymphocyte responses compared to SLA.

Resistance to the African Trypanosomes Is IFN-γ Dependent

The role of variant surface glycoprotein (VSG)-specific Th cell responses in determining resistance to the African trypanosomes was examined by comparing Th cell responses in relatively resistant and susceptible mice as well as in cytokine gene knockout mice infected with Trypanosoma brucei rhodesiense. Resistant B10.BR and C57BL/6 mice expressed Th1 cell cytokine responses to VSG stimulation during infection, while susceptible C3H mice produced weak or no Th1 cell cytokine responses. Neither resistant B10.BR and C57BL/6 mice nor susceptible C3H mice made detectable Th2 cell cytokine responses to parasite Ag. To more closely examine the potential role of IFN-γ and other cytokines in host resistance, we determined the resistance phenotypes and Th cell responses of IFN-γ and IL-4 knockout mice. Infected C57BL/6-IFN-γ knockout mice were as susceptible as C57BL/6-scid mice and made an IL-2, but not an IL-4, cytokine response to VSG, while C57BL/6-IL-4 knockout mice were as resistant as the wild-type strain and exhibited both IL-2 and IFN-γ cytokine responses. Passive transfer of spleen cells from wild-type mice to IFN-γ knockout mice resulted in enhanced survival. Both wild-type and IFN-γ knockout mice controlled parasitemia with VSG-specific Ab responses, although parasitemias were higher in the IFN-γ knockout mice. Overall, this study demonstrates for the first time that relative resistance to African trypanosomes is associated with a strong Th1 cell response to parasite Ags, that IFN-γ, but not IL-4, is linked to host resistance, and that susceptible animals do not make compensatory Th2 cell responses in the absence of Th1 cell cytokine responses.

Immunobiological studies on experimental visceral Leishmaniasis. V. The I-A(Bm12) mutation specifies resistance to infection

The I-A(Bm12) mutation of the I-A beta subunit converted Leishmania donovani-susceptible C57BL/6 (B6) mice into the relatively resistant B6C-H-2 Bm12 (Bm12) strain. The relative resistance was reflected not only in the reduced splenic and hepatic parasite burden in Bm12 (compared with B6) but also by the ability of Bm12 mice to mount a T-cell proliferative response to parasite antigens. Assay of antileishmanial antibody (immunoglobulin G (IgG)2a and IgG1) in the sera of infected mice showed that in Bm12 mice the predominant isotype was IgG2a, rather than IgG1, whereas a similar level of both isotypes were found in B6 mice. From the serum immunoglobulin isotype titre it appeared that the antileishmanial T-cell response was biased towards a T helper (Th) 1 response in Bm12 mice whereas it was a mixed Th1 and Th2 response in B6 mice. These observations provide credence to the notion that polymorphism in class II major histocompatibility complex (MHC) molecules is responsible for the difference in the disease phenotype.

Immunobiological studies on experimental visceral leishmaniasis. IV. Kinetics of evolution of disease-promoting versus host-protective cells of monocyte-macrophage lineage and their characterization

The evolution of cells of the monocyte-macrophage lineage (MML cells) in the spleen of Leishmania donovani (LD) infected BALB/c mice was studied. Spleen cells were fractionated on a discontinuous percoll gradient and adherent cells (AC) were purified from fractionated spleen cells by adherence steps that appeared at the interfaces of 25-35%, 35-40%, 40-45% and 45-50% percoll gradients. The AC were characterized as MML cells on the basis of positive staining for non-specific esterase. Adherent cells that appeared at the interfaces of 25-35% and 40-45% were defined as A and C, respectively, and both of them showed extreme variation in a progressive infection. It was observed that A supported parasite replication whereas C remained refractory when infected with LD in vitro. Furthermore, when A cells and C cells were used as antigen-presenting cells to stimulate mixed population of IFN-gamma producing and IL-4 producing T-cells, it was observed that IL-4 and IFN-gamma were the predominating cytokine in the T-cell supernatant, respectively. Both A and C were found to be increased hand-in-hand up to 5 months of infection and from then on A decreased and C increased in their numerical strength (A-C reciprocity). The evolution of A-C reciprocity coincided with the gradual reduction in the parasitaemia in the spleen suggesting that this may contribute to the acquisition of anti-leishmania immunity.

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.

Experimental murine paracoccidioidomycosis: relationship among the dissemination of the infection, humoral and cellular immune responses

The dissemination of Paracoccidioides brasiliensis cells to the heart, omentum/pancreas, spleen, liver and lungs, assessed by colony forming unit (CFU) counts, the levels of specific antibodies to this fungal agent (by ELISA), and the specific DTH reaction were studied in susceptible (B10.A) and resistant (A/Sn) mice. The animals were infected intraperitoneally with P. brasiliensis yeast cells and were evaluated 2, 4, 12 and 16 weeks later. The most remarkable differences between the two mouse strains were observed 16 weeks after infection, when B10.A mice displayed high numbers of CFU in all examined organs, except the heart, high antibody titres, and depressed DTH response. At this point, A/Sn mice presented low or absent CFU in all organs, low antibody titres and expressive DTH response. The CFU counts were shown to be a reliable parameter to discriminate susceptible from resistant animals. The fungal load in the most affected organs correlated with the antibody titres and was inversely correlated with the intensity of the DTH reaction. The patterns of immune response in this model mimic human paracoccidioidomycosis, in which high specific antibody levels and depressed DTH reactions are found in multifocal and severe forms of the disease.

Immunobiological studies on experimental visceral leishmaniasis. III. Cytokine-mediated regulation of parasite replication

The role of T-cell-derived cytokines in the regulation of Leishmania donovani replication was studied in a murine model. It was observed that in H-2d mice at the early and later stages of the disease IFN-gamma-secreting T cells predominate, whereas in between the above stages IL-4-secreting T cells predominate. Possibly, IL-4 abrogates the protective ability of IFN-gamma and thereby exponential parasite growth is ensured at the active stage of the disease. By contrast, H-2b mice were possibly incapable of inducing IL-4-secreting T cells and therefore parasite replication remains under control at any point post infection.

Activation of cytokine genes in T cells during primary and secondary murine influenza pneumonia

The patterns of cytokine mRNA expression in mice with primary or secondary influenza pneumonia have been assessed by in situ hybridization analysis of cells from both the mediastinal lymph node (MLN) and the virus-infected lung. Evidence of substantial transcriptional activity was found in all lymphocyte subsets recovered from both anatomical sites. The kinetics of cytokine mRNA expression after primary infection with an H3N2 virus were in accord with the idea that the initial response occurs in regional lymphoid tissue, with the effector T cells later moving to the lung. This temporal separation was much less apparent for the more rapid secondary response resulting from challenge of H3N2-primed mice with an H1N1 virus. Among the T cell receptor alpha/beta+ subsets, transcripts for interferon (IFN) gamma and tumor necrosis factor beta were most commonly found in the CD8+ population whereas mRNA for interleukin (IL) 4 and IL-10 was much more prevalent in CD4+ T cells. The gamma/delta T cells expressed mRNA for all cytokines tested, with IL-2, IL-4, and IFN-gamma predominating among those recovered from the inflammatory exudate. At particular time points, especially early in the MLN and late in the infected lung, the frequency of mRNA+ lymphocytes was much higher than would be expected from current understanding of the prevalence of virus-specific precursors and effectors. If this response is typical, induction of cytokine gene expression for T cells that are not responding directly to the invading pathogen may be a prominent feature of acute virus infections.

Immunobiological studies on experimental visceral leishmaniasis. II. Adherent cell-mediated down-regulation of delayed-type hypersensitivity response and up-regulation of B cell activation

Visceral leishmaniasis or kala-azar is characterized by a variety of immunopathological consequences in man. The most remarkable of these are the depression of cell-mediated immunity and polyclonal B cell activation. The consequences observed in man could be induced in a murine model by inoculating the causative agent, Leishmania donovani. The cell-mediated response was studied in this murine model in terms of the delayed-type hypersensitivity (DTH) response toward leishmania antigen in a progressive infection. BALB/b (H-2b) mice showed progressive enhancement in the DTH response, whereas BALB/c (H-2d) mice showed strong DTH at the onset which gradually disappeared (defined as DTH-negative phase) and reappeared again at the later stage of infection. Adoptive transfer of enriched populations of splenic T cells from infected BALB/c mice together with parasite antigen into the footpad of syngenic normal recipients produced a dramatic enhancement in the DTH response, except at the onset of the DTH-negative phase. These observations indicate that adherent cells have a role in suppression of the cell-mediated immune response and also that another mechanism operates at the onset of the DTH-negative phase. This DTH-negative phase was not caused by depletion of DTH-mediating cells from the repertoire, but rather by suppression mediated by a subset of T cell evolved in the course of infection. Characterization on the basis of lymphokine production of the T cells mediating the DTH response and of T cells mediating suppression of the DTH response showed them to be of Th1 and Th2 type, respectively. Studies also indicated that at the onset and the later stages of infection suppression was mediated by adherent cells, but at the onset of DTH-negative phase, in particular, suppression was mediated by Th2 cells. Furthermore, experiments also showed that adherent cells from infected mice gained another property, that of driving B cells, in a T cell-dependent manner.

Interferon-gamma inhibits the efficacy of interleukin 1 to generate a Th2-cell biased immune response induced by Leishmania major

Splenic adherent cells from L. major-infected resistant and susceptible mice were restimulated in vitro and analyzed for the expression of IL-1 activity. Three weeks or later after infection, cells from parasite infected susceptible BALB/c mice produced substantially more IL-1 activity than those from non-infected controls or from L. major-infected resistant C57BL/6 animals. More than 95% of the IL-1 bioactivity was mediated by IL-1 alpha, as determined by blocking experiments with an anti-IL-1 alpha antiserum. The strain-specific differences in IL-1 production correlated with different accumulation of IL-1 producing adherent cells in the spleens of infected animals, but also with different IL-1 producing capacity on a per cell basis. When adherent cells were mixed with syngeneic IFN-gamma producing CD4+ T lymphocytes from L. major-infected C57BL mice or from animals that had been pretreated with anti-CD4 monoclonal antibody prior to infection, the level of detectable IL-1 decreased depending on the number of T cells added. This inhibition could be blocked completely with an anti-IFN-gamma antibody. No such effect was seen, when CD4+ cells were used that were derived from parasite-infected BALB/c mice and did not produce IFN-gamma. In contrast to L. major, L. donovani antigen not only failed to induce IL-1 production, but also dose-dependently suppressed the IL-1 activity elaborated by L. major antigen. We conclude from these data that IFN-gamma effectively inhibits the efficacy to IL-1 to generate to Th2-cell biased immune response induced by L. major. A T cell independent and as yet unknown mechanism to inhibit the IL-1 response is used by L. donovani.

Immunobiological studies on experimental visceral leishmaniasis. I. Changes in lymphoid organs and their possible role in pathogenesis

Studies were carried out to determine changes in lymphoid organs i.e. spleen, lymph node and bone marrow (BM) in progressive experimental visceral leishmaniasis. Mononuclear phagocytes in the BM were increased; spleens showed a hypercellularity coupled with a rise in parasite burden while secondary follicles with no apparent depletion of paracortex were seen in the lymph node. This enhanced proliferation of mononuclear phagocytes in the BM and probably their subsequent recruitment in the spleen could be induced in naive recipients by injecting nylon wool-purified lymph node cells derived from infected mice together with sonicated leishmanial antigen(s). Similar changes could also be induced in the BM of naive recipients by injecting serum of infected mice. In control experiments where both donor and recipients were uninfected such changes were not apparent. A working hypothesis is proposed to delineate the role of lymphoid organs in the pathogenesis of experimental visceral leishmaniasis.

Immunization of susceptible hosts with a soluble antigen fraction from Leishmania major leads to aggravation of murine leishmaniasis mediated by CD4+ T cells

This study was performed in order to define Leishmania major antigens that function as disease-modulating immunogens in susceptible BALB/c mice. A soluble leishmanial antigen preparation (S-SLA) derived from highly infective stationary-phase L. major parasites was fractionated by preparative gel electrophoresis. In vitro, the low molecular mass fraction (less than 31 kDa) of S-SLA fraction D (FR D) was found to be a potent stimulator of L. major-specific Th1 and Th2 helper cell clones. In vivo, immunization with FR D induced a Th2-biased immune response in BALB/c mice as determined by the numbers of splenic CD4+ cells secreting interleukin 4 and interferon-gamma according to limiting dilution analyses. In addition, FR D caused significant disease exacerbation in parasite-infected susceptible mice as assessed by the local lesion development and the numbers of parasites in lymph nodes and spleen. This effect was observed after local subcutaneous application of FR D as well as after systemic immunization (intrasplenic or intraperitoneal). Transfer experiments revealed, that the disease-aggravating effect of FR D was mediated by CD4+ T cells. From these results it is concluded that leishmanial protein preparations exist that not only fail to induce protective anitparasitic immunity, but can mediate disease exacerbation, independently of the primary application site of the immunogen. The existence of such structures may serve the parasite as a means to evade the host's immune attack and may also have implications for the development of vaccines.

Evasion strategies of Leishmania parasites

Leishmania have long been known to clinicians and parasitologists as the causative agents of a variety of acute or chronic, cutaneous or visceral diseases in mammalian hosts. More recently, these protozoan parasites have evoked the interest of immunologists, as Leishmania infections are an excellent model for studying T-cell dominated antiparasite immune responses. In this review, Christian Bogdan, Martin Röllinghoff and Werner Solbach discuss the multiple interactions of Leishmania with components of the host immune system that illustrate the variety of highly elaborate evasion strategies developed by this parasite.