The immune system must respond rapidly and efficaciously to microbial infection, yet this response must avoid becoming pathogenic to the host. In this context, regulatory T cells (T regs; CD4+ CD25+, FoxP3 expressing T cells) play a central role in regulating adaptive immune responses to ensure that tolerance to host tissues is maintained. A case in point is that autoimmune disease develops in both humans and mice deficient in T regs. Conversely, depletion of T regs has been shown to augment the immune response to cancer. The regulatory action of T regs involves the cytokines IL-10 and TGF-?. Moreover, cell-to-cell contact appears to be required for the suppression of T cell proliferation in vitro. However, virtually nothing is known about the cellular specificity, stoichiometry, kinetics or location of immunoregulatory events in vivo. With recent advances in multi-photon microscopy, complex cellular interactions can now be studied directly in native tissues. Our approach is to use single-cell imaging to assess T reg impact on antigen presentation, T cell activation and effector function. We are currently developing two complementary mouse models for this purpose. With the first system we will investigate the role of T regs in the rejection or progression of implanted primary tumors. In the second model, we will examine the regulation of T cell responses during Listeria infection. Our hope is that comparing and contrasting T cell regulation in these systems will provide useful insight for how tolerance and immunity are governed. A secondary interest in the lab is to use biophysical data obtained by multi-photon microscopy to develop tissue level models for T cell regulation. The potential synergy between imaging and computational biology is tremendous i.e., computer simulations will guide experimental design, and in situ imaging will provide solid quantitative data for model refinement.