CD4 T cells are important for development of long-term immunity to bacterial infections. Here we describe construction of a group A streptococcus (GAS) strain that expresses the model ovalbumin epitope (OVA) on its surface, and the use of this strain in adoptive transfer experiments to study CD4 T cell response to bacterial infection in nasal-associated lymphoid tissue (NALT), which was previously shown to be a specific target for GAS colonization. The OVA + GAS, but not the wild-type strain was shown to activate CD4 T cells in an antigenspecific manner both in vitro and in vivo. After intranasal infection of mice with this strain, OVA-specific CD4 T cells were first activated in NALT, which is functionally equivalent to human tonsils, rather than in the cervical lymph nodes. During localized infection, OVA + GAS induced rapid and prolonged activation of CD4 T cells at higher magnitudes in the NALT than in draining lymph nodes and spleen, where CD4 T cells underwent little or no activation. In contrast, systemic infection induced significantly higher activation of CD4 T cells in both lymph nodes and spleens, compared to when the infection was localized in NALT. Further investigation of cellular immune responses in NALT during GAS infection using adoptive T cell transfer, combined with the model antigen on the pathogen may ultimately shed light on mechanisms for failure of children to develop protective immune responses following streptococcal tonsillitis.

Human ␥␦ T cells expressing the V␥9/V␦2 T-cell receptor have been previously found to proliferate in response to certain microorganisms and to expand throughout life, presumably because of extrathymic activation by foreign antigens. In vitro expansion of V␥9/V␦2 cells by mycobacteria has been previously shown to be dependent on accessory cells. In order to gain an insight into the mechanisms involved in the expansion of these cells, we have undertaken to identify the peripheral blood subset of cells on which proliferation of V␥9/V␦2 cells in response to mycobacteria is dependent. Contrary to their role in antigen presentation to ␣␤ T cells, professional antigen-presenting cells, such as monocytes, B cells, and dendritic cells, were unable to provide the cellular support for the expansion of V␥9/V␦2 cells. Selective depletion of T-cell subsets, as well as the use of highly purified T-cell populations, indicated that the only subset of peripheral blood cells that could expand V␥9/V␦2 cells were CD4 ؉ CD45RO ؉ CD7 ؊ ␣␤ T cells. These cells underwent distinct intracellular signaling events after stimulation with the mycobacterial antigen. Expansion of V␥9/V␦2 cells by ␣␤ T cells was dependent on cell-cell contact. This is the first evidence that a small subset of the memory helper T-cell population is exclusively responsible for the peripheral expansion of V␥9/V␦2 cells. These data illustrate a unique aspect of antigen recognition by ␥␦ T cells and provide new means to study their immune defense role.