T cell receptor-alpha beta-deficient mice fail to develop colitis in the absence of a microbial environment.

Mice with null mutations in cytokine or T cell receptor (TCR) genes develop intestinal inflammation. In the case of interleukin-2-/- and interleukin-10-/- mice it has been demonstrated that normal intestinal bacterial flora can cause gut pathology. TCR-alpha-/- mice not only develop colitis but also produce a strong antibody response to self-antigens, such as double-stranded DNA. It is therefore important to establish whether the intestinal inflammation develops spontaneously or is induced by luminal antigens. To address this issue, a germ-free colony of TCR-alpha-/- mice was derived and compared with TCR-alpha-/- mice kept in conventional specific-pathogen-free conditions. Although specific-pathogen-free animals developed colitis with a high level of penetrance, there was no evidence of intestinal pathology in germ-free animals. Furthermore, intestinal inflammation was not seen in TCR-alpha-/- mice colonized with a limited bacterial flora consisting of Lactobacillus plantarum, Streptococcus faecalis, S. faecium, and/or Escherichia coli. We conclude that intestinal inflammation in TCR-alpha-/- mice does not occur spontaneously nor does it result from the presence of bacteria, per se, but rather it is initiated by a specific organism or group of organisms normally present in the gut flora that have yet to be identified.

Germinal center formation in mice lacking alpha beta T cells.

T cells are essential for inducing clonal B cell expansion in germinal centers during T cell-dependent antibody responses. However, class-switched antibodies are readily detectable in TCR alpha-deficient mice that congenitally lack alpha beta T cells, including those such as IgG1 that are considered to be dependent on collaboration between B cells and alpha beta T cells. This observation suggests that a novel form of B:T collaboration may be evident in TCR alpha-/- mice. We report that germinal centers develop spontaneously in mice lacking T cell receptor alpha genes (TCR alpha-/-), despite the absence of alpha beta T cells. They are not seen in TCR beta-/- mice kept in similar conditions. Both strains of mice have gamma delta T cells, but it is a subset of T cells expressing TCR beta and CD4 that is dominant in the germinal centers of TCR alpha-/- mice. Exceptionally, germinal centers were associated with CD4+ gamma delta T cells. The expression of CD4 seems to be important, for few extrafollicular T cells have CD4 and CD4 is largely absent from TCR beta-/- T cells. The CD4+ TCR beta cells may help B cells produce autoantibodies that have been identified in TCR alpha-/- mice.

Germinal center formation, immunoglobulin class switching, and autoantibody production driven by "non alpha/beta" T cells.

The production of class-switched antibodies, particularly immunoglobulin (Ig) G1 and IgE, occurs efficiently in T cell receptor (TCR) alpha-/- mice that are congenitally devoid of alpha/beta T cells. This finding runs counter to a wealth of data indicating that IgG1 and IgE synthesis are largely dependent on the collaboration between B and alpha/beta T cells. Furthermore, many of the antibodies synthesized in TCR alpha-/- mice are reactive to a similar spectrum of self-antigens as that targeted by autoantibodies characterizing human systemic lupus erythematosus (SLE). SLE, too, is most commonly regarded as an alpha/beta T cell-mediated condition. To distinguish whether the development of autoantibodies in TCR alpha-/- mice is due to an intrinsic de-regulation of B cells, or to a heretofore poorly characterized collaboration between B and "non-alpha/beta T" cells, the phenotype has been reconstituted by transfer of various populations of B and non-alpha/beta T cells including cloned gamma/delta T cells derived from TCR alpha-/- mice, to severe combined immunodeficient (SCID) mice. The results establish that the reproducible production of IgG1 (including autoantibodies) is a product of non-alpha/beta T cell help that can be provided by gamma/delta T cells. This type of B-T collaboration sustains the production of germinal centers, lymphoid follicles that ordinarily are anatomical signatures of alpha/beta T-B cell collaboration. Thus, non-alpha/beta T cell help may drive Ig synthesis and autoreactivity under various circumstances, especially in cases of alpha/beta T cell immunodeficiency.

Lymphocyte proliferation in mice congenitally deficient in T-cell receptor alpha beta + cells.

In mice and humans, T cells are characterized on the basis of T-cell receptor (TcR) expression and divided into the major TcR alpha beta + and minor TcR gamma delta + populations. TcR alpha beta + cells are considered to be the primary regulators of the immune response, whereas the function of TcR gamma delta + cells is unclear. Mice congenitally deficient in TcR alpha beta-expressing cells provide an ideal model for analyzing the independent in vivo function of TcR gamma delta + cells in the absence of TcR alpha beta + cells. Here we report that lymphoid organs in TcR alpha mutant mice undergo substantial enlargement after being challenged by environmental antigens. This organ expansion can be attributed in part to increases in the relative proportions and absolute numbers of TcR gamma delta + cells, but an expansion of the recently described TcR beta + alpha - population also has a role. The expansion of the TcR gamma delta + population is polyclonal, as evidenced by the usage of multiple gamma and delta variable chain segments. Furthermore, a substantial proportion of the cells appears to be activated and these activated cells express surface activation markers. The results clearly demonstrate that TcR gamma delta + cells proliferate independently in response to a broad spectrum of challenges. Moreover, since the expansion of the lymphoid tissues and the TcR gamma delta + cell population is excessive relative to that seen in wild-type animals, one role of TcR alpha beta + cells is directly or indirectly to limit the responses of the other lymphoid components.

A "network antigen" for human CD4. A murine monoclonal anti-idiotype to Leu-3a induces an anti-CD4 response in naive mice.

Previous studies have evaluated anti-CD4 mAb as idiotypic models of the HIV gp120-binding site for CD4. The success of this strategy depends upon the concept of internal image, whereby the binding paratope of the anti-CD4 structurally mimics the equivalent binding surface on HIV gp120. To test this concept of internal image, anti-idiotypic antibodies were raised against the anti-CD4, Leu-3a. If any of these anti-Id detect the paratopic idiotope on the anti-CD4 antibody, their own respective paratopes should structurally model the corresponding binding epitope on CD4 bound by Leu-3a. Consequently, the immunization of naive mice with the selected anti-Id should induce an anti-CD4 response which reflects the binding specificities of Leu-3a. Four anti-Id to Leu-3a were characterized and tested for their ability to induce anti-CD4 responses in naive animals. Although one anti-Id induced an anti-CD4 response in mice, no such response could be detected in other species. Thus the failure to raise anti-Id with internal image characteristics may provide an explanation for the lack of anti-gp120 activity reported in anti-Id antisera raised to multiple anti-CD4 antibodies.

A highly selected panel of anti-CD4 antibodies fails to induce anti-idiotypic antisera mediating human immunodeficiency virus neutralization.

Anti-CD4 antibodies directed to the N terminus of CD4 can inhibit human immunodeficiency virus (HIV) infection. Therefore, it has been proposed that some of these reagents may contain idiotypic determinants which conformationally model the binding site expressed on gp120. In this report, we have selected a panel of anti-CD4 monoclonal antibodies as idiotypic mimics of gp120 by employing cross-blocking techniques, and CD4 epitope mapping using site-directed mutagenesis. These studies suggest that only 4 out of the original panel of 12 would be expected to represent suitable candidates for modelling the gp120 binding site. Nevertheless, anti-idiotypic antisera raised against these antibodies failed to inhibit gp120 binding to CD4. This negative result may reflect the incomplete modelling of the virus binding site by anti-CD4, or the lack of internal image antibody in the anti-idiotypic preparations. Alternatively, the binding site on gp120 may not be accessible to antibody neutralization, excluding the possibility of an idiotypic vaccine to HIV based on anti-CD4 antibody as surrogate antigen.

Novel anti-CD4 monoclonal antibodies separate human immunodeficiency virus infection and fusion of CD4+ cells from virus binding.

Human immunodeficiency virus (HIV) binds to cells via an interaction between CD4 and the virus envelope glycoprotein, gp120. Previous studies have localized the high affinity binding site for gp120 to the first domain of CD4, and monoclonal antibodies (mAbs) reactive with this region compete with gp120 binding and thereby block virus infectivity and syncytium formation. Despite a detailed understanding of the binding of gp120 to CD4, little is known of subsequent events leading to membrane fusion and virus entry. We describe two new mAbs reactive with the third domain of CD4 that inhibit steps subsequent to virus binding critical for HIV infectivity and cell fusion. Binding of recombinant gp120 or virus to CD4 is not inhibited by these antibodies, whereas infection and syncytium formation by a number of HIV isolates are blocked. These findings demonstrate that in addition to virus binding, CD4 may have an active role in membrane fusion.