Brian Lee Kelsall, M.D., B.A.

Antigen Presentation by Mucosal DCs and the Regulation of Mucosal Immune Responses

Stimulation of T helper (Th) cells and IgA precursor B cells in the Peyer's patches (PP) of the small intestine with orally administered antigens leads to the dissemination of B and Th cells to mucosal effector tissues, such as the lamina propria of the GI and upper respiratory tracts, and to secretory glands for subsequent antigen-specific secretory IgA antibody responses.

At the same time, however, systemic T- and B-cell immune responses to the same antigen may be suppressed--a phenomenon known as "oral tolerance." This ability of oral antigens to both stimulate mucosal and suppress systemic immune responses likely involves antigen processing and presentation in the PP/lymphoid follicles of the intestine, which have associated epithelial cells (M cells), specialized for the sampling and transport of luminal antigens.

The main objective of our work to date has been to understand how protein antigens are processed and presented in the murine PP and how this relates to oral tolerance and IgA B-cell development.

Major Accomplishments

• We have identified two different populations of DCs in the PP by immunohistochemical staining, one of which is densely concentrated in the subepithelial dome and is poised to capture antigens transported into the PP by overlying M cells. We have demonstrated that PP DCs express higher levels of MHC class II antigens than DCs from the spleen, suggesting that DCs in the PP are in a more activated state and may be able to provide a higher affinity interaction with T cells. In addition, we showed that in cognate interactions with T cells from TCR-transgenic mice with a B10 background, when compared to spleen DCs, PP DCs produce higher levels of IL-12, which results in higher levels of IFN-g production or a skewing of the Th-cell response to Th1 T cells.
• We have shown that the feeding of high dose ovalbumin (OVA) to OVA-TCR-transgenic mice results in the priming of PP T cells for both IFN-g production and enhanced apoptosis upon restimulation in vitro, while spleen T cells are anergic to proliferation and produce suppressed levels of Th0-like cytokines (i.e., tolerized). By blocking IL-12 with anti-IL-12 administered systemically at the time of antigen feeding, systemic tolerance was enhanced, and this was due to both an increase in T-cell apoptosis and the induction of TGF-ß-producing cells that suppress in vitro proliferation of non-tolerized T cells. This ability of anti-IL-12 to augment oral tolerance has implications for the treatment of autoimmune disease with oral antigens, and suggests that IL-12 may be a major regulator of TGF-ß production.
• We have recently described the determinants of TGFß production by T cells following both in vivo immunization and in vitroprimary and secondary T-cell stimulation. We have determined that the absence of IL-12 and IFN-g is important for the production of TGF-ß during a primary stimulation in vitro and that the absence of IFN-g and IL-12 and the presence of IL-4 are important for priming for secondary TGF-ß responses.

Future Directions

• Define the phenotype and function of subpopulations of Peyer’s patch dendritic cells using freshly isolated cells from normal mice.
• Demonstrate the applicability of systemic anti-IL-12 and oral antigen feeding to the treatment to an animal model of an autoimmune disease, experimental allergic encephalitis (EAE).

Regulation of IL-12 Production

A second research interest of the laboratory has been the regulation of IL-12 production from antigen presenting cells, such as DCs and macrophages. Since IL-12 is important in shaping Th-cell phenotype, knowledge of what regulates IL-12 production is important for the manipulation of immune responses with vaccines and immunomodulators. The main objective of this work to date has been to identify and define both positive and inhibitory regulators of IL-12 production.

Major Accomplishments

• We have demonstrated that DCs produce IL-12 in response to signaling through the CD40 molecule on the surface of DCs. As DCs are the primary cell responsible for initiating T-cell responses, the interaction of CD40 with CD40L on activated T cells may be of major importance in understanding T-cell differentiation in vivo.

• We have determined that the complement receptor CR3 (CD11b/CD18, Mac-1) is involved in the regulation of IL-12 production by human macrophages. This is of significant interest because CR3 acts as a receptor not only for the iC3b component of complement that opsonizes microorganisms, but also as a direct receptor for several intracellular pathogens, such as Histoplasma capsulatum and Leishmania species, and for ICAM-1, an important molecule involved in cell-cell interactions.

• We have now defined the ability of cholera toxin, a potent mucosal adjuvant, to suppress IL-12 production from human monocytes and DCs in vitro, as well as to suppress IL-12 responses in mice to systemic challenge with lipopolysaccharide. This effect was also seen with a related ADP-ribosylating toxin, heat-labile toxin from E. coli. These studies have implications for the development of adjuvants for mucosal immune responses, as well as for the development of novel treatments for Th1-dependent autoimmunity.

Future Directions

• Elucidate the mechanisms by which signaling through CR3 and by cholera toxin result in enhanced or suppressed IL-12 production by human macrophages and DCs.
• Explore the use of ADP-ribosylating toxins and mutant toxins in the treatment Th1-mediated inflammatory bowel disease models.

Dr. Kelsall received his B.A. in human biology from Stanford University in 1982. In 1986, he earned his M.D. from Case Western Reserve University School of Medicine. He did postdoctoral training in internal medicine at The New York Hospital-Cornell Medical Center from 1986 to 1989 and in infectious diseases at the University of Virginia Medical Center from 1989 to 1992.

In 1992, Dr. Kelsall came to the National Institutes of Health, completed fellowship training in mucosal immunology in 1996, and became a senior investigator in 2003. His research focuses on the regulation of immune responses in the intestine, in particular the role that unique intestinal dendritic cell and macrophage populations play in the induction of immunity to intestinal viral pathogens and mucosal vaccines and in the pathogenesis of inflammatory bowel disease.