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Development of NKT cells was shown to depend on lymphotoxin (LT) and IL-15 signaling pathways as well as on cytokine receptor common gamma chain. After positive selection, NKT-cell precursors transit through progressive maturation stages including proliferative expansion within the NK1.1(-) window. The transcription factors that integrate different signaling pathways into different stages of NKT-cell development are not well characterized. Here, we show that the Rel/NF-kappaB family member RelA regulates the NK1.1(-) to NK1.1(+) transition during NKT-cell development. RelA is also required for both IL-15- and IL-7-induced proliferation of CD44(hi)NK1.1(-) NKT-cell precursors. Activation of the invariant NKT-cell receptor induces both IL-15 receptor alpha and gamma chains' expression in an NF-kappaB-dependent manner, suggesting a molecular mechanism by which NF-kappaB regulates NKT-cell development. NF-kappaB also regulates TCR-induced expression of LT-alpha and LT-beta within NKT cells. In contrast to previous reports, however, we show that LT signaling is dispensable for thymic NKT-cell development but is essential for their colonization of peripheral organs such as liver.

In the early phase of leishmaniasis three types of potential antigen-presenting cells, including epidermal Langerhans cells (LC), dermal dendritic cells (DC) and inflammatory DC, are localized at the site of infection. Therefore, it has been a central question which cell type is responsible for the initiation of a protective immune response. In the early stage of an anti-Leishmania immune response, detectable Leishmania major antigen was localized in the paracortex of the draining lymph nodes (LN). Characterization of antigen-positive cells showed that L. major co-localized with DC of a CD11c(+) CD8 alpha(-) Langerin(-) phenotype. To determine the area of antigen uptake, dermis or epidermis, and to further define the type of antigen-transporting cells, L. major was inoculated subcutaneously and concurrently LC were mobilized with fluorescein isothiocyanate (FITC). After 3 days, DC carrying L. major antigen were always FITC(-), indicating a dermal and not an epidermal origin. Moreover, addition of L. major antigen to ex vivo isolated CD8 alpha(-) and CD8 alpha(+) DC from the draining LN of L. major-infected C57BL/6 mice demonstrated that both DC subpopulations were able to stimulate antigen-specific T cell proliferation in vitro. Without addition of exogenous antigen only the CD8 alpha(-) Langerin(-) DC were capable of stimulating antigen-specific T cell proliferation. Thus, we demonstrate that CD8 alpha(-) Langerin(-) DC and not LC are the basis of the protective immune response to intracellular L. major parasites in vivo.

Activation of resting T cells usually leads to their proliferation and differentiation into effector cells and a subsequent decline following elimination of the antigen. A situation of excessive antigen density may result in T cell receptor (TCR)-induced deletion of T effector cells, a process termed antigen-induced cell death (AgICD). Previous studies indicate that AgICD of cytotoxic T cells may be induced by either of the two key cytotoxic processes, granule exocytosis, including perforin and granzymes, or the Fas ligand (FasL)/Fas pathway. By using in vitro-polyclonally activated or ex vivo-derived virus-induced T cell populations from mice with mutations or targeted gene defects in one or more components of the two key cytolytic pathways we now show that TCR-induced apoptosis is only impaired in the absence of FasL and/or Fas, but not in the absence of perforin and/or granzymes. Furthermore, antibody-blockage of FasL alone is sufficient to inhibit early T cell death. Inhibition of both, FasL and tumor necrosis factor (TNF-alpha) is required to abrogate late apoptosis by AgICD. The fact that antibodies to IFN-gamma also inhibit AgICD suggests that the perforin plus granzyme-independent and FaSL and/or TNF-alpha facilitated process of AgICD of T effector cells is tightly regulated by endogenous IFN-gamma.

TNF receptor-ligand interactions and CD95 (Fas / APO-1) have been demonstrated to be involved in activation-induced death of mature T cells. Here, we examined the role of these molecules in the murine model of lymphocytic choriomeningitis virus (LCMV) infection using LCMV TCR transgenic (tg) mice lacking TNF, TNF receptor I (TNFR1), CD95 or both TNFR1 and CD95. This report demonstrates that neither TNF receptor-ligand interactions nor CD95 was required for down-regulation of LCMV-specific CD8 T cells following acute LCMV infection in vivo. Even LCMV-specific CD8 T cells lacking both TNFR1 and CD95 molecules declined after the acute phase of the infection with normal kinetics. Furthermore, peripheral deletion of LCMV-specific CD8 T cells induced by LCMV peptide injection or by adoptive transfer of tg spleen cells expressing the corresponding LCMV epitope was not impaired in mice lacking TNF, TNFR1 and / or CD95. Our data speak against an indispensable role of these molecules in antigen-induced apoptosis of CD8 T cells in vivo and suggest that T cell homeostasis after antigen challenge is controlled by additional mechanisms.

The mechanisms by which UV radiation is immunosuppressive are controversial, but there is growing evidence that processes of UVB-induced suppression of the immune response towards a sensitizing antigen are different if this antigen is applied to irradiated compared with non-irradiated sites. Consistent with this is our recent observation (Hart et al., J. Exp. Med. 1998. 187: 2045-2053) that the prevalence of dermal mast cells determines the extent of susceptibility of different mouse strains to UVB-induced systemic, but not local, immunosuppression. Using C57BL/6 and BALB/c mice exposed to low and high doses of UVB, respectively, in the presence of a polyclonal anti-TNF antibody, we found that TNF is directly involved as a mediator in the suppression by UVB of local immune responses. To determine whether TNF indirectly regulates UVB-induced systemic immunomodulation by altering the prevalence of dermal mast cells, dermal mast cell numbers in gene-targeted mice deficient in TNF or TNF receptors (p55/p75-/- mice) were quantified by video image analysis. A reduced dermal mast cell prevalence in these mice correlated with decreased susceptibility for systemic immunosuppression caused by UVB. We hypothesize that TNF is one molecule that controls dermal mast cell prevalence by as yet unknown mechanisms. However, it is the mediators released from mast cells upon UVB-induced degranulation, which do not include TNF, that directly signal suppressive events relevant to systemic immunosuppression.

Specialized roles for the pro-inflammatory cytokines tumor necrosis factor (TNF) and lymphotoxin (LT) were characterized in TNF/LT alpha -/- and TNF -/- mice established by direct gene targeting of C57BL/6 ES cells. The requirement for LT early in lymphoid tissue organogenesis is shown to be distinct from the more subtle and varied role of TNF in promoting correct microarchitectural organization of leukocytes in LN and spleen. Development of normal Peyer's patch (PP) structure, in contrast, is substantially dependent on TNF. Only mice lacking LT exhibit retarded B cell maturation in vivo and serum immunoglobulin deficiencies. A temporal hierarchy in lymphoid tissue development can now be defined, with LT being an essential participant in general lymphoid tissue organogenesis, developmentally preceeding TNF that has a more varied and subtle role in promotion of correct spatial organization of leukocytes in LN and spleen PP development in TNF -/- mice is unusual, indicating that TNF is a more critical participant for this structure than it is for other lymphoid tissues.

Recently, we demonstrated that experimental autoimmune encephalomyelitis (EAE) in the rat, passively transferred using myelin basic protein (MBP)-reactive encephalitogenic CD4+ T cells, was preventable by administration of a p55-tumor necrosis factor-IgG fusion protein (TNFR-IgG). This was despite quantitatively and qualitatively normal movement of these MBP-specific T cells to the central nervous system (CNS). To extend these findings, the effect of TNFR-IgG on EAE actively induced by injection of MBP in complete Freund's adjuvant was examined. This form of EAE in the rat typically involves an acute, self-limiting neurological deficit, substantial CNS inflammation, but minimal demyelination. Here we show that administration of TNFR-IgG prior to onset of disease signs completely prevented the neurological deficit or markedly reduced its severity. This blockade of clinical disease was dissociated from weight loss which occurred at the same tempo and magnitude as in control rats exhibiting neurological signs of disease such as paralysis. The timing of TNF blockade was critical as established clinical disease was relatively refractory to TNFR-IgG treatment. Activated CD4+ T cells expressing normal or elevated levels of VLA4, major histocompatibility complex class II, MRC OX40 and CD25 were isolated from or immunohistochemically localized in the CNS of clinically healthy rats treated before disease onset. There was a reduction of the amount of other inflammatory leukocytes in the CNS of these treated animals but, more importantly, the activation state of inflammatory leukocytes, as well as that of microglia isolated from treated animals, was reduced. Thus, TNFR-IgG, when administered before disease onset, appears to act by inhibiting an effector function of activated T cells and possibly other inflammatory leukocytes necessary to bring about the neurological deficit. However, while TNF is a critically important cytokine for the early events leading to initiation of EAE, it is not a necessary factor in the acute neurological deficit characteristic of this form of EAE, once disease onset has occurred.

Activation of resting T cells usually leads to their proliferation and differentiation into effector cells and a subsequent decline following elimination of the antigen. A situation of excessive antigen density may result in T cell receptor (TCR)-induced deletion of T effector cells, a process termed antigen-induced cell death (AgICD). Previous studies indicate that AgICD of cytotoxic T cells may be induced by either of the two key cytotoxic processes, granule exocytosis, including perforin and granzymes, or the Fas ligand (FasL)/Fas pathway. By using in vitro-polyclonally activated or ex vivo-derived virus-induced T cell populations from mice with mutations or targeted gene defects in one or more components of the two key cytolytic pathways we now show that TCR-induced apoptosis is only impaired in the absence of FasL and/or Fas, but not in the absence of perforin and/or granzymes. Furthermore, antibody-blockage of FasL alone is sufficient to inhibit early T cell death. Inhibition of both, FasL and tumor necrosis factor (TNF-alpha) is required to abrogate late apoptosis by AgICD. The fact that antibodies to IFN-gamma also inhibit AgICD suggests that the perforin plus granzyme-independent and FaSL and/or TNF-alpha facilitated process of AgICD of T effector cells is tightly regulated by endogenous IFN-gamma.

Lymphotoxin (LT)alpha in combination with LTbeta forms membrane-bound heterotrimeric complexes with a crucial function in lymph node (LN) organogenesis and correct morphogenesis of secondary lymphoid tissue. To study the role of membrane LT (mLT) in lymphoid tissue organogenesis we generated an LTbeta-deficient mouse strain on a pure genetic C57BL/6 background (B6.LTbeta-/-) and compared it to a unique series of LTalpha-, TNF- and TNF/LTalpha-gene-targeted mice on an identical genetic background (B6.LTalpha-/-, B6.TNF-/- and B6 TNF/LTalpha-/-). B6.LTbeta-/- mice lacked peripheral LN with the exception of mesenteric LN, and displayed a disturbed micro-architecture of the spleen, although less profoundly than LTalpha- or TNF/LTalpha-deficient mice. Radiation bone marrow chimeras (B6.WT--B6.LTbeta-/- developed Peyer's patch (PP)-like lymphoid aggregates in the intestinal wall indicating a possible role for soluble LTalpha(3) in the formation of the PP anlage. After infection with Leishmania major, B6.LTbeta-/- mice developed a fatal disseminating leishmaniasis resulting in death after 8 to 14 weeks, despite the natural resistance of the C57BL/6 genetic background (B6.WT) mice to the parasite. Both, the cellular and the humoral anti-L. major immune responses were delayed and ineffective. However, the expression pattern of the key cytokines IFN-gamma and IL-12 were comparable in B6.WT and B6.LTbeta-/- mice. Infection of radiation bone marrow chimeras showed that it is the LTbeta-dependent presence of lymphoid tissue and not the expression of mLT itself that renders mice resistant to leishmaniasis.

Recent studies demonstrated that administration of a p55-tumor necrosis factor (TNF) receptor IgG-fusion protein (TNFR-IgG) prevented the clinical onset of experimental autoimmune encephalomyelitis but did not alter the number or tissue distribution of autoantigen-specific CD4+ effector T cells which trafficked into the central nervous system. To determine whether specific target tissues of autoimmune damage remain intact after TNFR-IgG treatment despite the presence of inflammatory cells within the tissues, we examined rats with experimental autoimmune uveoretinitis (EAU), as in this model, the main target of autoreactive CD4+ T cells, the retinal rod outer segments (ROS), can be examined readily by light microscopy. As judged by direct ophthalmoscopy, the onset of inflammation in the anterior chamber of the eye in EAU following administration of TNFR-IgG was delayed by 6 days compared to untreated controls, but the magnitude of the response was only slightly less than controls. Histological examination of the retinae and direct assessment of retinal inflammation revealed a disproportionate sparing of ROS in the TNFR-IgG-treated animals despite a level of retinal inflammation not substantially less than controls in which ROS damage was marked. Analysis of retinal leukocytes by immunofluorescence microscopy and flow cytometry indicated that approximately equal numbers of CD4+ alpha beta TCR+ lymphocytes were present in treated and control retinae, more than 30% of CD4+ cells in both experimental groups expressed the CD25 or MRC OX40 activation markers and most cells, which would include the CD4+ T lymphocytes, were activated as evidenced by MHC class II expression. Fewer activated macrophages and granulocytes were present in the treated retinae, possibly reflecting the lower level of tissue damage and subsequent accumulation of these inflammatory cells. The results demonstrate directly that a tissue specifically targeted for autoimmune destruction can be protected despite the influx of fully activated CD4+ T cells.

Lymphotoxin (LT)alpha in combination with LTbeta forms membrane-bound heterotrimeric complexes with a crucial function in lymph node (LN) organogenesis and correct morphogenesis of secondary lymphoid tissue. To study the role of membrane LT (mLT) in lymphoid tissue organogenesis we generated an LTbeta-deficient mouse strain on a pure genetic C57BL/6 background (B6.LTbeta-/-) and compared it to a unique series of LTalpha-, TNF- and TNF/LTalpha-gene-targeted mice on an identical genetic background (B6.LTalpha-/-, B6.TNF-/- and B6 TNF/LTalpha-/-). B6.LTbeta-/- mice lacked peripheral LN with the exception of mesenteric LN, and displayed a disturbed micro-architecture of the spleen, although less profoundly than LTalpha- or TNF/LTalpha-deficient mice. Radiation bone marrow chimeras (B6.WT-->B6.LTbeta-/- developed Peyer's patch (PP)-like lymphoid aggregates in the intestinal wall indicating a possible role for soluble LTalpha(3) in the formation of the PP anlage. After infection with Leishmania major, B6.LTbeta-/- mice developed a fatal disseminating leishmaniasis resulting in death after 8 to 14 weeks, despite the natural resistance of the C57BL/6 genetic background (B6.WT) mice to the parasite. Both, the cellular and the humoral anti-L. major immune responses were delayed and ineffective. However, the expression pattern of the key cytokines IFN-gamma and IL-12 were comparable in B6.WT and B6.LTbeta-/- mice. Infection of radiation bone marrow chimeras showed that it is the LTbeta-dependent presence of lymphoid tissue and not the expression of mLT itself that renders mice resistant to leishmaniasis.