Eric G. Pamer, MD

Mice infected with L. monocytogenes develop a rapid inflammatory response that is followed by a robust and highly protective T cell mediated immune response. The early inflammatory response is essential for survival and inflammatory monocyte recruitment to sites of infection is central to this process (Figure 1). Our laboratory is characterizing the recruitment of inflammatory monocytes from the bone marrow into the bloodstream and infected tissues following systemic infection with L. monocytogenes. We have found that the CCR2 chemokine receptor is essential for monocyte emigration from the bone marrow and that cellular infection with L. monocytogenes induces the production of MCP-1, the ligand for CCR2. We have generated novel mouse strains that enable us to detect in vivo chemokine expression and inflammatory monocyte trafficking within infected tissues.

Figure 1 Inflammatory monocytes (in green) are recruited to foci of Listeria infection in the spleen. Focal induction of ICAM-1 expression in the liver (in red) targets monocytes to sites of bacterial infection.

Figure 2 Phase contrast image of two Clostridium difficile strains that differ in virulence undergoing sporulation.

A second focus of our laboratory concerns mucosal immune defense against intestinal pathogens such as Clostridium difficile and Vancomycin-resistant Enterococcus (VRE). These pathogens inhabit the intestine and are important causes of infection in hospitalized patients. Infection by these organisms generally occurs following antibiotic treatment and antibiotic-mediated disturbance of the normal intestinal microbiota enhances susceptibility to these pathogens (Figure 2). We are using these models to dissect the mechanisms by which commensal bacteria enhance resistance to microbial pathogens. We have demonstrated that antibiotic-mediated depletion of commensal bacteria decreases the expression of antimicrobial factors by the intestinal mucosa, enabling organisms like VRE to thrive and invade the bloodstream. Our laboratory is developing new approaches to enhance mucosal resistance in a variety of clinical circumstances where the intestinal microbiota is disturbed (e.g. following allogeneic hematopoietic stem cell transplantation).

Our laboratory also investigates the impact of antibiotics on the composition of the intestinal microbiota, using the Roche-454 multiparallel pyrosequencing platform (Figure 3). Our laboratory has demonstrated that antibiotics induce dramatic and long-term shifts of the intestinal microbiota that markedly enhance susceptibility to colonization and infection with VRE and Clostridium difficile.

Enlarge Image Figure 3

Our laboratory collaborates with the laboratory of Michael Glickman to investigate CD4 T cell responses to Mycobacterium tuberculosis. We have generated a T cell receptor transgenic mouse strain specific for the immunodominant ESAT-6 epitope. T cells from this mouse provide resistance to M. tuberculosis infection, enabling us to perform experiments to optimize CD4 T cell-mediated protection and to discover and enhance mechanisms of protective immunity to tuberculosis.