Our Science – Segal Website

David M. Segal, Ph.D.

Experimental Immunology Branch Head, Immune Targeting Section Senior Investigator Building 10, Room 4B36 10 Center Drive Bethesda, MD 20892 Phone:   301-496-3109 Fax:   301-496-0887 E-Mail:   dave_segal@nih.gov Link: Other Homepage

Biography

Dr. Segal received a B.A. from Oberlin College and a Ph.D. from Johns Hopkins University. After two years as a postdoctoral fellow at the Weizmann Institute he came to the NIH where he studied the structures of antibodies and other proteins in the Laboratory of Molecular Biology, NIDDK. He then joined the NCI's Immunology Branch to pursue an interest in immune recognition. Recently Dr. Segal's research has focused on molecules of the innate immune system, particularly the Toll-like receptors.

Research

Toll Like Receptors and Innate Immunity
My laboratory investigates the structure and function of Toll-like Receptors (TLRs) and their role in innate and acquired immunity. The vertebrate immune response to infection begins with the recognition by the innate immune system of conserved molecular signatures of pathogens, known as PAMPs (Pathogen Associated Molecular Patterns), provoking an immediate and often massive inflammatory response. The innate response holds the pathogen in check, but also plays a crucial role in the generation of acquired immunity. The recognition of PAMPs by the innate system is mediated by a number of receptors, of which the Toll-like Receptors (TLRs) play a prominent role. Unlike the antigen receptors of acquired immunity, the TLRs are encoded by a limited number of germline genes, ten in humans; however, in spite of their small numbers, the TLRs recognize a remarkably wide variety of PAMPs including glycolipids, proteins, and nucleic acids. The molecular basis for the recognition of PAMPs by TLRs is a main interest of my laboratory. In collaboration with the Davies lab (LMB, NIDDK), we have expressed mg amounts of the extracellular domain (ECD) of TLR3, and have determined its structure by X-ray crystallography. Double-stranded (ds)RNA, a molecular signature of many viruses, activates TLR3. In solution, purified TLR3-ECD binds dsRNA specifically via a defined ligand-binding site, with an affinity that increases with buffer acidification and ligand size. TLR3-ECD forms dimers when bound to dsRNA and the smallest oligonucleotides that form stable complexes with TLR3-ECD (40-50 bp) are also the smallest dsRNAs that activate TLR3 in cells. We have solved the structures of both unliganded TLR3-ECD, and the dimeric complex with dsRNA (see picture in gallery). The structure nicely explains the functional properties of TLR3, including specificity, length of dsRNA requirements, pH dependence, and mode of signaling. Point mutation analyses have shown three low affinity TLR3 binding sites act cooperatively to form a stable signaling complex.
TLRs play a pivotal role in acquired immunity by triggering the maturation of dendritic cells (DC) to competent antigen presenting cells (APC), capable of priming naive T cells. Because TLR3 triggers a unique activation pathway, we hypothesized that antigen-bearing DC activated by dsRNA would generate an acquired response different from DC activated with other PAMPs. Based upon our previous observations that the capacity of different length dsRNA oligonucleotides to activate TLR3 depended upon levels of TLR3 expression, we are performing experiments to determine how DCs respond to different length dsRNA oligos, and how these DC activate naive T cells, especially in comparison with the TLR9 PAMP, CpG DNA. An overall goal of these studies is to fine-tune PAMP adjuvants to generate immune responses specific for particular classes of pathogens. In the case of dsRNA, a viral PAMP, we would expect the response to be especially effective against viral pathogens.

This page was last updated on 10/22/2009.