Mark Bix

Dynamic regulatory network controlling Th17 cell differentiation

Despite their importance, the molecular circuits that control the differentiation of naive T cells remain largely unknown. Recent studies that reconstructed regulatory networks in mammalian cells have focused on short-term responses and relied on perturbation-based approaches that cannot be readily applied to primary T cells. Here we combine transcriptional profiling at high temporal resolution, novel computational algorithms, and innovative nanowire-based perturbation tools to systematically derive and experimentally validate a model of the dynamic regulatory network that controls the differentiation of mouse TH17 cells, a proinflammatory T-cell subset that has been implicated in the pathogenesis of multiple autoimmune diseases. The TH17 transcriptional network consists of two self-reinforcing, but mutually antagonistic, modules, with 12 novel regulators, the coupled action of which may be essential for maintaining the balance between TH17 and other CD4+ T-cell subsets. Our study identifies and validates 39 regulatory factors, embeds them within a comprehensive temporal network and reveals its organizational principles; it also highlights novel drug targets for controlling TH17 cell differentiation.

Susceptibility to infectious diseases: Leishmania as a paradigm.

The diverse response of individuals within populations to infectious pathogens remains poorly understood, although genetic determinants undoubtedly contribute in substantial ways to the outcome of infection. In a mouse model of infection with the intramacrophage protozoan Leishmania major, susceptibility correlates both with aberrant helper T cell differentiation biased towards the production of interleukin 4 and with the presence of an endogenous CD4 T cell repertoire that recognizes an immunodominant parasite antigen with high frequency. In the setting of the particular ecological niche occupied by Leishmania, this combination of otherwise unrelated factors synergizes to result in exquisite susceptibility to this single pathogen, without seemingly compromising host defenses against other agents. Similar paradigms could underlie susceptibility to other pathogenic organisms.

Cutting Edge: Dab2 Is a FOXP3 Target Gene Required for Regulatory T Cell Function

FOXP3-expressing regulatory T (Treg) cells are vital for maintaining peripheral T cell tolerance and homeostasis. The mechanisms by which FOXP3 target genes orchestrate context-dependent Treg cell function are largely unknown. In this study we show that in mouse peripheral lymphocytes the Drosophila Disabled-2 (Dab2) homolog, a gene that is involved in enhancing TGFβ responses, is exclusively expressed in FOXP3+ regulatory T cells. Dab2 is a direct target of FOXP3, and regulatory T cells lacking DAB2 are functionally impaired in vitro and in vivo. However, not all aspects of Treg cell function are perturbed, and DAB2 appears to be dispensable for Treg cell function in maintaining naive T cell homeostasis.

Development of CD4+ effector T cells and susceptibility to infectious diseases.

The mechanisms by which naive helper T cells differentiate into potent cytokine-expressing effectors remain critical to understanding both successful and aberrant immune responses. Studies using Leishmania major infection of mice have revealed genetic contributions to factors that influence this differentiative process. Further, antigen recognition at the level of the T cell repertoire can also profoundly affect the outcome of disease and the appearance of discrete T cell subsets. It is likely that such mechanisms also underpin genetic susceptibility to diverse other infectious and autoimmune diseases.

The Development of Effector T Cell Subsets in Murine Leishmania Major Infection

Leishmania major infection has proven an exceptional model for CD4+ subset development in inbred mice. Most strains contain infection coincident with the appearance of T helper 1 (Th1) cells that produce gamma-interferon (IFN-gamma) required for macrophage activation. In contrast, mice on the BALB background are unable to control infection due to the development of Th2 cells that produce counter-regulatory cytokines, particularly interleukin 4 (IL-4), capable of abrogating the effects of IFN-gamma. Selective gene disruption studies in mice have illustrated critical components of the host response to L. major. Mice deficient in beta 2 microglobulin, which have no major histocompatibility complex (MHC) class I or CD8+ T cells, control infection as well as wild-type mice, whereas mice deficient in MHC class II (and CD4+ T cells) suffer fatal infection. Mice with disruption of the gene coding IFN-gamma are also incapable of containing infection, reflecting absolute requirements for this cytokine. A number of interventions have been demonstrated to abrogate Th2 cell development in BALB mice, enabling these mice to control infection. Each of these--IL-12, anti-IL-4, anti-IL-2, anti-CD4 and CTLA4-Ig--has in common the capacity to make IL-4 rate limiting at the time of CD4+ cell priming.

Lymphocyte Development in Mice Deficient for MHC Class I Expression

The role of MHC Class I molecules in recognition of antigens by CD8+ T cells is well established, as is their role in the development of CD8+ T cells. The study of animals unable to normally express class I molecules provides an approach to learn about the role of class I molecules not only in CD8+ T cell development, but possibly in other cell types as well. This can be accomplished by analysis of mice mutant for the light chain of class I MHC, β2-microglobulin, which is necessary for normal functional cell surface expression of class I molecules. Such mice were produced in two laboratories by substitution of the normal β2-microglobulin gene for a mutant one, by homologous recombination embryonic stem cells, which were allowed to repopulate the germ line in chimeric mice 1, 2, 3, 4. In previous studies, we and others reported that mice homozygous for the mutant β2-m gene have severely diminished cell surface expression of MHC-I molecules, and are severely deficient in the production of functional, mature CD8+CD4- T cells 2,4. In this article, we summarize our recent work on the role of MHC-I molecules in development of CD8+ T cells, CD4-CD8-∝β+ T cells, and γδ+ T cells. In addition the role of MHC-I molecules in the development of natural killer (NK) cells is discussed.