Surface lymphotoxin alpha/beta complex is required for the development of peripheral lymphoid organs.

For more than a decade, the biological roles and the apparent redundancy of the cytokines tumor necrosis factor (TNF) and lymphotoxin (LT) have been debated. LT alpha exists in its soluble form as a homotrimer, which like TNF only binds the TNF receptors, TNF-R55 or TNF-R75. The cell surface form of LT exists as a heteromer of LT alpha and LT beta subunits and this complex specifically binds the LT beta receptor (LT beta-R). To discriminate the functions of the LT and TNF systems, soluble LT beta-R-immunoglobulin (Ig) or TNF-R-Ig fusion proteins were introduced into embryonic circulation by injecting pregnant mice. Exposure to LT beta-R-Ig during gestation disrupted lymph node development and splenic architecture in the progeny indicating that both effects are mediated by the surface LT alpha/beta complex. These data are the first to identify a cell surface ligand involved in immune organ morphogenesis. Moreover, they unambiguously discriminate the functions of the various TNF/LT ligands, provide a unique model to study compartmentalization of immune responses and illustrate the generic utility of receptor-Ig fusion proteins for dissecting/ordering ontogenetic events in the absence of genetic modifications.

Hapten-induced colitis is associated with colonic patch hypertrophy and T helper cell 2-type responses.

To investigate the potential involvement of T helper (Th)2-type responses in murine models of intestinal inflammation, we used trinitrobenzene sulfonic acid (TNBS)-hapten to induce inflammatory bowel disease in situations where Th1-type responses with interferon (IFN)-gamma synthesis are either diminished or do not occur. Intracolonic administration of TNBS to either normal (IFN-gamma+/+) or Th1-deficient IFN-gamma knockout (IFN-gamma-/-) BALB/c mice resulted in significant colitis. In IFN-gamma-/- mice, crypt inflammation was more severe than in IFN-gamma+/+ mice and was accompanied by hypertrophy of colonic patches with a lymphoepithelium containing M cells and distinct B and T cell zones resembling Peyer's patches. Hapten-specific, colonic patch T cells from both mouse groups exhibited a Th2 phenotype with interleukin (IL)-4 and IL-5 production. TNBS colitis in normal mice treated with anti-IL-4 antibodies or in IL-4(-/-) mice was less severe than in either IFN-gamma+/+ or IFN-gamma-/- mice. Our findings now show that the Th2-type responses in TNBS colitis are associated with colonic patch enlargement and inflammation of the mucosal layer and may represent a model for ulcerative colitis.

Essential role of nuclear factor (NF)-kappaB-inducing kinase and inhibitor of kappaB (IkappaB) kinase alpha in NF-kappaB activation through lymphotoxin beta receptor, but not through tumor necrosis factor receptor I.

Both nuclear factor (NF)-kappaB-inducing kinase (NIK) and inhibitor of kappaB (IkappaB) kinase (IKK) have been implicated as essential components for NF-kappaB activation in response to many external stimuli. However, the exact roles of NIK and IKKalpha in cytokine signaling still remain controversial. With the use of in vivo mouse models, rather than with enforced gene-expression systems, we have investigated the role of NIK and IKKalpha in signaling through the type I tumor necrosis factor (TNF) receptor (TNFR-I) and the lymphotoxin beta receptor (LTbetaR), a receptor essential for lymphoid organogenesis. TNF stimulation induced similar levels of phosphorylation and degradation of IkappaBalpha in embryonic fibroblasts from either wild-type or NIK-mutant mice. In contrast, LTbetaR stimulation induced NF-kappaB activation in wild-type mice, but the response was impaired in embryonic fibroblasts from NIK-mutant and IKKalpha-deficient mice. Consistent with the essential role of IKKalpha in LTbetaR signaling, we found that development of Peyer's patches was defective in IKKalpha-deficient mice. These results demonstrate that both NIK and IKKalpha are essential for the induction of NF-kappaB through LTbetaR, whereas the NIK-IKKalpha pathway is dispensable in TNFR-I signaling.

Essential role of lymph nodes in contact hypersensitivity revealed in lymphotoxin-alpha-deficient mice.

Lymph nodes (LNs) are important sentinal organs, populated by circulating lymphocytes and antigen-bearing cells exiting the tissue beds. Although cellular and humoral immune responses are induced in LNs by antigenic challenge, it is not known if LNs are essential for acquired immunity. We examined immune responses in mice that lack LNs due to genetic deletion of lymphotoxin ligands or in utero blockade of membrane lymphotoxin. We report that LNs are absolutely required for generating contact hypersensitivity, a T cell-dependent cellular immune response induced by epicutaneous hapten. We show that the homing of epidermal Langerhans cells in response to hapten application is specifically directed to LNs, providing a cellular basis for this unique LN function. In contrast, the spleen cannot mediate contact hypersensitivity because antigen-bearing epidermal Langerhans cells do not access splenic white pulp. Finally, we formally demonstrate that LNs provide a unique environment essential for generating this acquired immune response by reversing the LN defect in lymphotoxin-alpha(-/)- mice, thereby restoring the capacity for contact hypersensitivity.

Regulation of peripheral lymph node genesis by the tumor necrosis factor family member TRANCE.

Proper lymph node (LN) development requires tumor necrosis factor-related activation-induced cytokine (TRANCE) expression. Here we demonstrate that the defective LN development in TRANCE(-/)- mice correlates with a significant reduction in lymphotoxin (LT)alphabeta(+)alpha(4)beta(7)(+)CD45(+)CD4(+)CD3(-) cells and their failure to form clusters in rudimentary mesenteric LNs. Transgenic TRANCE overexpression in TRANCE(-/)- mice results in selective restoration of this cell population into clusters, and results in full LN development. Transgenic TRANCE-mediated restoration of LN development requires LTalphabeta expression on CD45(+) CD4(+)CD3(-) cells, as LNs could not be induced in LTalpha(-/)- mice. LTalpha(-/)- mice also showed defects in the fate of CD45(+)CD4(+)CD3(-) cells similar to TRANCE(-/)- mice. Thus, we propose that both TRANCE and LTalphabeta regulate the colonization and cluster formation by CD45(+) CD4(+)CD3(-) cells in developing LNs, the degree of which appears to correlate with the state of LN organogenesis.

Regulation of inflammation by collagen-binding integrins alpha1beta1 and alpha2beta1 in models of hypersensitivity and arthritis.

Adhesive interactions play an important role in inflammation by promoting leukocyte attachment and extravasation from the vasculature into the peripheral tissues. However, the importance of adhesion molecules within the extracellular matrix-rich environment of peripheral tissues, in which cells must migrate and be activated, has not been well explored. We investigated the role of the major collagen-binding integrins, alpha1beta1 and alpha2beta1, in several in vivo models of inflammation. mAb's against murine alpha1 and alpha2 were found to significantly inhibit effector phase inflammatory responses in animal models of delayed-type hypersensitivity (DTH), contact hypersensitivity (CHS), and arthritis. Mice that were alpha1-deficient also showed decreased inflammatory responses in the CHS and arthritis models when compared with wild-type mice. Decreased leukocyte infiltration and edema formation accompanied inhibition of antigen-specific models of inflammation, as nonspecific inflammation induced by croton oil was not inhibited. This study demonstrates the importance in vivo of alpha1beta1 and alpha2beta1, the collagen-binding integrins, in inflammatory diseases. The study also extends the role of integrins in inflammation beyond leukocyte attachment and extravasation at the vascular endothelial interface, revealing the extracellular matrix environment of peripheral tissues as a new point of intervention for adhesion-based therapies.

Nerve growth factor mRNA in brain: localization by in situ hybridization.

Nerve Growth Factor is a 118 amino acid polypeptide that plays an important role in the differentiation and survival of neurons. The recent discovery that a mRNA that encodes beta Nerve Growth Factor is present in brain suggests that the Nerve Growth Factor gene may not only regulate gene expression of peripheral but also of central neurons. To identify the site(s) of Nerve Growth Factor mRNA production in the brain and to determine which cells express the Nerve Growth Factor gene, the technique of in situ hybridization was employed. A 32P-labeled RNA probe complementary to Nerve Growth Factor mRNA hybridized to cells in the stratum granulosum of the dentate gyrus and the stratum pyramidale of the hippocampus. These observations identify for the first time cellular sites of Nerve Growth Factor gene expression in the central nervous system, and suggest that Nerve Growth Factor mRNA is produced by neurons.

Selective disruption of lymphotoxin ligands reveals a novel set of mucosal lymph nodes and unique effects on lymph node cellular organization.

Lymphotoxin (LT) provides a critical signal for the genesis of lymph nodes (LN) in mice. Here we show that mice treated in utero with LT beta-R-Ig, which binds to the membrane LT alpha 1 beta 2 heterotrimer, lacked most LN, yet retained a set of mucosal surface draining LN. Since mice genetically deficient in LT alpha lack all LN, including the mucosal set, we hypothesize that a novel LT alpha-dependent pathway controls their genesis. This novel set of mucosal LN cannot be discriminated on the basis of addressin expression. The discovery of LN in mice treated with LT beta-R-Ig fusion protein in utero allowed us to compare the roles of membrane LT alpha beta or soluble LT alpha/tumor necrosis factor (TNF) in the development of cellular organization in LN and spleen. Our results indicate that both membrane LT alpha beta and soluble LT alpha/TNF mediate T-B cell segregation and the organization of B cell follicles in spleen and LN. Interestingly, while antagonism of membrane LT alpha beta or soluble LT alpha/TNF prevented germinal center (GC) formation in spleen, antagonism of soluble LT alpha/TNF had no effect on LN formation. The data suggest that multiple LT/TNF ligands control B cell follicle organization in the spleen and LN of adult mice, and that the requirements for LT/TNF ligands in GC formation are distinct in the different lymphoid organs.

Lymph node genesis is induced by signaling through the lymphotoxin beta receptor.

We investigated lymphotoxin (LT) and TNF function in lymph node genesis and cellular organization by manipulating LTbeta-R and TNF-R signaling. Lymph nodes developed in LTalpha-/- mice treated in utero with agonist anti-LTbeta-R monoclonal antibody. Thus, LTbeta-R signaling mediates lymph node genesis. Surprisingly, mucosal lymph nodes that can develop independently of LTalphabeta/LTbeta-R interaction were generated. Normal mice treated in utero with LTbeta-R-Ig and TNF-R55-Ig or anti-TNF lacked all lymph nodes, indicating that TNF signaling contributes to lymph node genesis. Lymph nodes generated in LTalpha-/- mice had disrupted cellular organization. Therefore, LTbeta-R signaling during gestation is not sufficient to establish normal cellular microarchitecture. We conclude that LT and TNF play critical roles in the genesis and cellular organization of lymph nodes.

Peyer's patches are required for oral tolerance to proteins.

To clarify the role of Peyer's patches in oral tolerance induction, BALB/c mice were treated in utero with lymphotoxin beta-receptor Ig fusion protein to generate mice lacking Peyer's patches. When these Peyer's patch-null mice were fed 25 mg of ovalbumin (OVA) before systemic immunization, OVA-specific IgG Ab responses in serum and spleen were seen, in marked contrast to low responses in OVA-fed normal mice. Further, high T-cell-proliferative- and delayed-type hypersensitivity responses were seen in Peyer's patch-null mice given oral OVA before systemic challenge. Higher levels of CD4(+) T-cell-derived IFN-gamma, IL-4, IL-5, and IL-10 syntheses were noted in Peyer's patch-null mice fed OVA, whereas OVA-fed normal mice had suppressed cytokine levels. In contrast, oral administration of trinitrobenzene sulfonic acid (TNBS) to Peyer's patch-null mice resulted in reduced TNBS-specific serum Abs and splenic B cell antitrinitrophenyl Ab-forming cell responses after skin painting with picryl chloride. Further, when delayed-type hypersensitivity and splenic T cell proliferative responses were examined, Peyer's patch-null mice fed TNBS were unresponsive to hapten. Peyer's patch-null mice fed trinitrophenyl-OVA failed to induce systemic unresponsiveness to hapten or protein. These findings show that organized Peyer's patches are required for oral tolerance to proteins, whereas haptens elicit systemic unresponsiveness via the intestinal epithelial cell barrier.

Mutations in the principal neutralization determinant of human immunodeficiency virus type 1 affect syncytium formation, virus infectivity, growth kinetics, and neutralization.

The principal neutralization determinant (PND) of human immunodeficiency virus type 1 envelope glycoprotein gp120 contains a conserved GPG sequence. The effects of a 29-amino-acid deletion of most of the PND, a 3-amino-acid deletion in the GPG sequence, and 16 single-amino-acid substitutions in the GPG sequence were determined in a transient expression assay. All mutant envelope glycoproteins were expressed at levels comparable to that of the wild-type envelope, and mutations in the GPG sequence did not affect processing to gp120 or, except for the 29-amino-acid deletion, binding to CD4. Of all of the mutants, only the GHG and GFG mutants induced formation of syncytia similar in size and number to those induced by the wild-type envelope. When the envelope expression level was increased 10-fold or more, several additional mutants (APG, GAG, GSG, GQG, GVG, and GPF) also induced syncytium formation. Transfection with infectious proviral molecular clones containing the GHG, GFG, APG, GAG, GSG, or GPF mutations induced production of viral particles; however, only the GPG, GHG, and GFG viruses produced active infections in CD4-bearing cells. Furthermore, whereas the wild-type virus was efficiently neutralized by PND polyclonal and monoclonal antibodies, the GHG- and GFG-containing viruses were not. These results show that mutations in the GPG sequence found within the PND do not affect envelope expression and do not significantly affect CD4 binding or production of viral particles but that they do affect the ability of the envelope to induce syncytia and those of the viral particles to infect CD4 cells and be neutralized by PND antibodies.

Elimination of colonic patches with lymphotoxin beta receptor-Ig prevents Th2 cell-type colitis.

Past studies have shown that colonic patches, which are the gut-associated lymphoreticular tissues (GALT) in the colon, become much more pronounced in hapten-induced murine colitis, and this was associated with Th2-type T cell responses. To address the role of GALT in colonic inflammation, experimental colitis was induced in mice either lacking organized GALT or with altered GALT structures. Trinitrobenzene sulfonic acid was used to induce colitis in mice given lymphotoxin-beta receptor-Ig fusion protein (LTbetaR-Ig) in utero, a treatment that blocked the formation of both Peyer's and colonic patches. Mice deficient in colonic patches developed focal acute ulcers with Th1-type responses, whereas lesions in normal mice were of a diffuse mucosal type with both Th1- and Th2-type cytokine production. We next determined whether LTbetaR-Ig could be used to treat colitis in normal or Th2-dominant, IFN-gamma gene knockout (IFN-gamma(-/-)) mice. Four weekly treatments with LTbetaR-Ig resulted in deletion of Peyer's and colonic patches with significant decreases in numbers of dendritic cells. This pretreatment protected IFN-gamma(-/-) mice from trinitrobenzene sulfonic acid-induced colitis; however, in normal mice this weekly treatment was less protective. In these mice hypertrophy of colonic patches was seen after induction of colitis. We conclude that Th2-type colitis is dependent upon the presence of colonic patches. The effect of LTbetaR-Ig was mediated through prevention of colonic patch hypertrophy in the absence of IFN-gamma. Thus, LTbetaR-Ig may offer a possible treatment for the Th2-dominant form of colitis.

Targeted disruption of the zetaPKC gene results in the impairment of the NF-kappaB pathway.

Here we have addressed the role that zetaPKC plays in NF-kappaB activation using mice in which this kinase was inactivated by homologous recombination. These mice, although grossly normal, showed phenotypic alterations in secondary lymphoid organs reminiscent of those of the TNF receptor-1 and of the lymphotoxin-beta receptor gene-deficient mice. The lack of zetaPKC in embryonic fibroblasts (EFs) severely impairs kappaB-dependent transcriptional activity as well as cytokine-induced phosphorylation of p65. Also, a cytokine-inducible interaction of zetaPKC with p65 was detected which requires the previous degradation of IkappaB. Although in zetaPKC-/- EFs this kinase is not necessary for IKK activation, in lung, which abundantly expresses zetaPKC, IKK activation is inhibited.

Involvement of distinct cellular compartments in the abnormal lymphoid organogenesis in lymphotoxin-alpha-deficient mice and alymphoplasia (aly) mice defined by the chimeric analysis.

Both lymphotoxin-alpha (LTalpha)-deficient mice and alymphoplasia (aly) mice, a natural mutant strain, manifest a quite similar phenotype: lack of lymph nodes (LN) and Peyer's patches (PP), with disturbed spleen architecture. The mechanisms underlying the defective lymphoid organogenesis in these mice were investigated by generating aggregation chimeras; ex vivo fused morulae were implanted into pseudo-pregnant host females and allowed to develop to term. Chimeric mice between LTalpha-deficient mice and wild-type mice restored LN and PP almost completely, suggesting that LTalpha expressed by circulating bone marrow-derived cells is essential for lymphoid organogenesis as well as for organization of spleen architecture. By contrast, chimeric mice between aly mice and wild-type mice showed only limited restoration of LN and PP. This suggests that the putative aly gene product does not act as a circulating ligand for lymphoid organogenesis, like LTalpha. Rather, abnormal development of lymphoid organs in aly mice seems most likely due to the defective development of the incipient stromal cells of the LN and PP. Supporting this hypothesis, up-regulation of VCAM-1 on aly mouse embryonic fibroblasts by signals through LTbetaR, which is exclusively expressed by nonlymphoid cells, was disturbed. These studies demonstrate that LTalpha and the putative aly gene product together control lymphoid organogenesis with a close mechanistic relationship in their biochemical pathways through governing the distinct cellular compartments, the former acting as a circulating ligand and the latter as a LTbetaR-signaling molecule expressed by the stroma of the lymphoid organs.

Induction of oral tolerance to cellular immune responses in the absence of Peyer's patches.

Systemic hyporesponsiveness occurs following oral administration of antigen (oral tolerance) and involves the uptake and processing of antigen by the gut-associated lymphoid tissue (GALT), which includes Peyer's patches (PP) lamina propria lymphocytes and mesenteric lymph nodes (MLN). Animals with targeted mutations of genes in the tumor necrosis factor (TNF) family have differential defects in the development of peripheral lymphoid organs including PP and MLN, and provide a unique opportunity to investigate the role of GALT structures in the induction of oral tolerance. Oral tolerance could not be induced in TNF/lymphotoxin (LT) alpha-/- mice, which are devoid of both PP and MLN, although these animals could be tolerized by intraperitoneal administration of antigen, demonstrating the requirement for GALT for oral tolerance induction. LTbeta-/- mice and LTalpha/LTbeta+/- animals do not have PP but could be orally tolerized, as measured by IFN-gamma production and delayed-type hypersensitivity responses by administration of both low or high doses of ovalbumin. To further investigate the requirement for PP, we tested the progeny of LTbeta-receptor-IgG-fusion-protein (LTbetaRigG)-treated mice, which do not form PP but have an otherwise intact immune system. Although these animals had decreased fecal IgA production, they could be orally tolerized. Our results demonstrate that PP are not an absolute requirement for the induction of either high- or low-dose oral tolerance, although oral tolerance could not be induced in animals devoid of both PP and MLN.

B7-1 and B7-2 do not deliver identical costimulatory signals, since B7-2 but not B7-1 preferentially costimulates the initial production of IL-4.

The functional necessity for two CD28 counterreceptors (B7-1 and B7-2) is presently unknown. B7-1 and B7-2 equivalently costimulate IL-2 and interferon-gamma (IFN gamma) production and IL-2 receptor alpha and gamma chain expression. B7-2 induces significantly more IL-4 production than B7-1, with the greatest difference seen in naive T cells. Repetitive costimulation of CD4+ CD45RA+ T cells with B7-2 results in moderate levels of both IL-4 and IL-2, whereas repetitive costimulation with B7-1 results in high levels of IL-2 and low levels of IL-4. Therefore, B7-1 and B7-2 costimulation mediate distinct outcomes, since B7-2 provides an initial signal to induce naive T cells to become IL-4 producers, thereby directing the immune response more towards Th0/Th2, whereas B7-1 is a more neutral differentiative signal.

Characterization of lymphotoxin-alpha beta complexes on the surface of mouse lymphocytes.

The lymphotoxin-alpha beta complex (LT alpha beta) is found on the surface of activated lymphocytes and binds to a specific receptor called the LT beta receptor (LT beta R). In the mouse, signaling through this pathway is important for lymph node development and splenic organization, yet the biochemical properties of murine LT alpha and LT beta are essentially unknown. Here we have used soluble receptor-Ig forms of LT beta R and TNF-R55 and mAbs specific for murine LT alpha, LT beta, and LT beta R to characterize the appearance of surface LT alpha beta complexes and LT beta R on several common murine cell lines. Cells that bound LT beta R also bound anti-LT alpha and anti-LT beta mAbs in a FACS analysis. The ability of these reagents to discriminate between surface TNF and LT was verified by analysis of surface TNF-positive, LPS-activated murine RAW 264.7 monocytic cells. Primary mouse leukocytes from spleen, thymus, lymph node, and peritoneum were activated in vitro, and CD4+ and CD8+ T cells as well as B cells expressed surface LT ligand but not the LT beta R. Conversely, elicited peritoneal monocytes/macrophages were surface LT negative yet LT beta R positive. This study shows that on mononuclear cells, surface LT complexes and receptor are expressed similarly in mice and man, and the tools described herein form the foundation for study of the functional roles of the LT system in the mouse.

Targeted disruption of Traf5 gene causes defects in CD40- and CD27-mediated lymphocyte activation.

TRAF5 [tumor necrosis factor (TNF) receptor-associated factor 5] is implicated in NF-kappaB and c-Jun NH(2)-terminal kinase/stress-activated protein kinase activation by members of the TNF receptor superfamily, including CD27, CD30, CD40, and lymphotoxin-beta receptor. To investigate the functional role of TRAF5 in vivo, we generated TRAF5-deficient mice by gene targeting. Activation of either NF-kappaB or c-Jun NH(2)-terminal kinase/stress-activated protein kinase by tumor necrosis factor, CD27, and CD40 was not abrogated in traf5(-/-) mice. However, traf5(-/-) B cells showed defects in proliferation and up-regulation of various surface molecules, including CD23, CD54, CD80, CD86, and Fas in response to CD40 stimulation. Moreover, in vitro Ig production of traf5(-/-) B cells stimulated with anti-CD40 plus IL-4 was reduced substantially. CD27-mediated costimulatory signal also was impaired in traf5(-/-) T cells. Collectively, these results demonstrate that TRAF5 is involved in CD40- and CD27-mediated signaling.