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Dendritic cells (DCs) generated from bone marrow (BM) precursor cells of C57BL/6 (B6.WT) mice and cultured in the presence of granulocyte macrophage-colony stimulating factor differentiate to mature BM-DCs spontaneously. These mature DCs are characterized by high levels of major histocompatibility complex (MHC) class II, CD40, and CD86 on their surface. To analyze the involvement of tumor necrosis factor (TNF) and the related cytokine lymphotoxin (LT)alpha in DC maturation, we studied the development of DCs from the BM of B6.TNF(-/-), B6.LTalpha(-/-), and B6.TNF/LTalpha(-/-) mice and compared it to B6.WT mice. Although the development of BM precursor cells to the level of immature DCs (CD11c(+), MHC class II(low), CD40(low), and CD86(low)) was equivalent in all genotypes, B6.TNF(-/-) and B6.TNF/LTalpha(-/-) cells showed an impaired capacity to differentiate to mature DCs. In contrast, mature BM-DCs generated from LTalpha-negative, immature DCs developed like B6.WT cells. Further studies revealed that once matured, the phenotype of all tested genotypes was comparable. They expressed high levels of CD40 and CD86, were exclusively positive for the chemokine receptor (CCR)7 but negative for CCR5 and CCR2, and were able to enter the paracortex of draining lymph nodes. The limited maturation of TNF-deficient BM-DCs could be restored by mixing TNF-negative with TNF-positive Ly5.1 BM cells, and maturation of B6.WT DCs could be blocked with an anti-TNF monoclonal antibody. The substitution of B6.TNF(-/-) BM cells with recombinant TNF revealed promotion or suppression of BM-DC maturation depending on the point of time of TNF addition.

Immunity to Toxoplasma gondii critically depends on TNFR type I-mediated immune reactions, but the precise role of the individual ligands of TNFR1, TNF and lymphotoxin-alpha (LTalpha), is still unknown. Upon oral infection with T. gondii, TNF(-/-), LTalpha(-/-), and TNF/LTalpha(-/-) mice failed to control intracerebral T. gondii and succumbed to an acute necrotizing Toxoplasma encephalitis, whereas wild-type (WT) mice survived. Intracerebral inducible NO synthase expression and-early after infection-splenic NO levels were reduced. Additionally, peritoneal macrophages produced reduced levels of NO upon infection with T. gondii and had significantly reduced toxoplasmastatic activity in TNF(-/-), LTalpha(-/-), and TNF/LTalpha(-/-) mice as compared with WT animals. Frequencies of parasite-specific IFN-gamma-producing T cells, intracerebral and splenic IFN-gamma production, and T. gondii-specific IgM and IgG titers in LTalpha(-/-) and TNF/LTalpha(-/-) mice were reduced only early after infection. In contrast, intracerebral IL-10 and IL-12p40 mRNA expression and splenic IL-2, IL-4, and IL-12 production were identical in all genotypes. In addition, TNF(-/-), LTalpha(-/-), and TNF/LTalpha(-/-), but not WT, mice succumbed to infection with the highly attenuated ts-4 strain of T. gondii or to a subsequent challenge infection with virulent RH toxoplasms, although they had identical frequencies of IFN-gamma-producing T cells as compared with WT mice. Generation and infection of bone marrow reconstitution chimeras demonstrated an exclusive role of hematogeneously produced TNF and LTalpha for survival of toxoplasmosis. These findings demonstrate the crucial role of both LTalpha and TNF for control of intracerebral toxoplasms.

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.

Lymphotoxin (LT)-alpha, a member of the TNF family, is recognized as an important mediator in different aspects of lymphoid organ development. Targeted disruption of this molecule resulted in a substantial reduction in the proportion of alphaEbeta7-integrin(high) CD8+ T cells detectable in peripheral lymphoid organs. This defect, however, was not observed on mature CD4-CD8+ thymocytes. To determine whether this was due to downregulation of beta7-integrin expression by peripheral CD8+ T cells or a failure of thymic emigration of CD8+ beta7-integrin(high) T cells, beta7-integrin was examined on recent thymic emigrants (RTE). When analysed within 16 h after leaving the thymus CD4-CD8+ RTE in both LT-alpha-/- and wild type (wt) mice remained beta7-integrin(high) and were indistinguishable. However, within 3-5 days, emigration loss of beta7-integrin became evident in LT-alpha-/- mice. Despite this loss, the proportion of thymically derived alphabetaTCR+ T-cell populations in the intestinal epithelium, an important target tissue of CD8+ alphaEbeta7-integrin(high) T cells, was increased in the absence of LT-alpha. In contrast, B cells were detectable only rarely in the intestinal tissue of LT-alpha-/- mice. The expression of E-Cadherin remained unchanged. These results indicate that a LT-alpha-dependent process maintains a high level of alphaEbeta7-integrin expression by peripheral CD8+ T cells, and with this control mechanism LT-alpha may help to regulate CD8+ T-cell numbers in the tissues.

In normal spleen, most recirculating naïve IgM+IgDhi B cells are located within primary follicles and mantle zones of secondary follicles. By contrast, the marginal zone contains a heterogeneous population of IgMhiIgDlo/- B cells that are mostly non-recirculating. Although these are dynamic populations they are maintained at a constant size, the fundamental homeostatic mechanisms remain uncertain. One possibility is that the presence and turnover of each of the B cell populations is dependent on their location within discrete splenic compartments. To investigate this, we have characterized immature, non-recirculating, mature recirculating, marginal zone and B-1 cell populations in TNF-/- and TNF/lymphotoxin(LT)-alpha-/- mice that have disorganized splenic architecture. Labelling with 5-bromo-2'-deoxyuridine revealed that turnover of B cells in TNF-/- mice is normal, but is diminished in TNF/LT-alpha-/- mice. The recirculating B cell populations in both mutant strains are normal in proportion and phenotype. Marginal zone B cells are not seen in TNF/LT-alpha-/- mice, but this population appears normal in TNF-/- mice, even though they lack germinal centres. These findings indicate that peripheral B cell subsets can be established and maintained independently of normal follicular architecture.

Mice infected with an adenovirus mutant in which the E3 region is deleted, including TNF-resistance genes, develop fatal liver pathology within 3-4 days after infection. At least 10-fold more wild-type virus was needed to cause comparable pathology. These results indicate that the E3 region is critically involved in modulating the pathogenesis of adenovirus infection and that TNF may play a role in liver damage. To explore the latter possibility, the course of disease was examined in infected mice lacking TNFR-I and/or TNFRII, TNF only, or both TNF and lymphotoxin-alpha. Only mice lacking both TNFRI and TNFRII were protected from the lethal affects of the mutant adenovirus. Mice deficient in TNF or TNF and lymphotoxin-alpha displayed the fatal pathology. This outcome is consistent with the existence of another related ligand that binds TNFRI/II to mediate liver damage during infection with this mutant.

To evaluate the importance of the proinflammatory cytokines TNF and lymphotoxin-alpha (LT alpha) in an experimental model of Staphylococcus aureus sepsis and arthritis, we used TNF/LT alpha-double-deficient mice raised on the C57BL/6 background. Mice were i.v. inoculated with a toxic shock syndrome toxin-1 (TSST-1)-producing S. aureus strain, LS-1. Intravenous inoculation of a high dose of bacteria (1 x 10(7)/mouse) resulted in 67% mortality in TNF/LT alpha-deficient mice, whereas none of the controls died (p = 0.009). Those results correlated to a significantly decreased phagocytosis in vitro and inefficient bacterial clearance in vivo in mice lacking capacity to produce TNF/LT alpha. Thus, at day 6 after inoculation, S. aureus could not be found in the bloodstream of controls, but bacteremia developed in all TNF/LT alpha-deficient mice examined (p = 0.02). Interestingly, upon infection with a lower dose of staphylococci (3 x 10(6)/mouse) the mortality was overall low, but the frequency of arthritis was clearly higher in the wild-type group as compared with the TNF/LT alpha-deficient mice (40% vs 13%). Histopathologic examination revealed a lower frequency of synovitis (38% vs 90%, p < 0.05) and erosivity (25% vs 60%, NS) in TNF/LT alpha-deficient mice as compared with wild-type counterparts. Our results show the importance of TNF/LT alpha in defense against systemic S. aureus infections and point out the detrimental role of these cytokines as mediators of inflammatory response in S. aureus arthritis.

Lymphotoxin (LT) is widely regarded as a proinflammatory cytokine with activities equivalent to tumor necrosis factor (TNF). The contribution of LT to experimental autoimmune encephalomyelitis (EAE) was examined using TNF/LTalpha-/- mice, TNF-/- mice, and a new LTalpha-/- line described here. All mice were generated directly in the C57BL/6 strain and used for the preparation of radiation bone marrow chimeras to reconstitute peripheral lymphoid organs and restore immunocompetence. This approach overcame the problems related to the lack of lymph nodes that results from LTalpha gene targeting. We show here that when LT is absent but TNF is present, EAE progresses normally. In contrast, when TNF is absent but LT is present, EAE is delayed in onset and inflammatory leukocytes fail to move normally into the central nervous system parenchyma, even at the peak of disease. In the absence of both cytokines, the clinical and histological picture is identical to that seen when TNF alone is deficient, including demyelination. Furthermore, the therapeutic inhibition of TNF and LTalpha with soluble TNF receptor in unmanipulated wild-type or TNF-/- mice exactly reproduces these outcomes. We conclude from these studies that TNF and LT are functionally distinct cytokines in vivo, and despite sharing common receptors, show no redundancy of function nor mutual compensation.

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.

Tumour necrosis factor (TNF) is a highly potent, proinflammatory cytokine with broad-ranging functions from the regulation of endothelial cell adhesion molecules to facilitate entry of leucocytes into tissues, to direct induction of cellular cytotoxicity. This diversity of function potentially attributable to TNF in the genesis of inflammatory disorders place TNF as a primary candidate for clinical targeting and considerable success in this regard has been achieved, particularly in rheumatoid arthritis (RA). In this article we provide a short overview of TNF and its homologue lymphotoxin (LT) alpha and beta. Particular emphasis is placed on recent discoveries regarding the cell surface expression of these cytokines and the role of TNF/LT in experimental autoimmune encephalomyelitis (EAE), an animal model of the human demyelinating disease, multiple sclerosis (MS).