Antigen-specific competitive inhibition of CD4+ T cell recruitment into the primary immune response

Previous studies suggest that recruitment of naive T cells into a program of cell division and differentiation is a highly synchronous process under tight regulation. However it is not known whether antigen availability is the major regulator of this process, or whether other factors such as ongoing responses to unrelated antigens can affect the size of the primary response. We have developed an adoptive transfer system to investigate the efficiency with which additional antigen specific cells are recruited into an ongoing primary immune response. Recruitment of additional cells is an inverse function of the size of the response and is progressively inhibited with time. Cells recruited late into the response proliferate less, and fewer secrete IL-2 and IFN-{gamma}. Thus the size of the response changes very little as a result of late recruitment. The inhibition of recruitment, proliferation and differentiation affects only cells of the same specificity as the ongoing response, indicating that the size of an antigen specific response is independent of any shared factors such as access to APCs, costimulation or cytokines. Thus, during infection, the immune system retains the ability to respond as necessary to secondary infections or antigens not presented until later stages of the response.