Limiting dilution analysis of proliferating and helper T cells in the in vivo immune response to KLH: derepression of helper T cells at moderately increased frequencies.

Abstract

While it is clear that some T cells have the capacity for almost indefinite proliferation in vitro, it is a controversial issue how much of this proliferative capacity is utilized by T cells in a response to antigen in vivo. In the framework of a strict clonal selection model the functional activities of normal and immune lymphocyte populations are essentially determined by the frequencies of antigen-specific cells which are clonally expanded after recognition of antigen. In contrast, our group has proposed a network model in which the more prominent effect of immunization is a release of antigen-specific T cells from a state of suppression (= derepression) which exists in the non-immune situation and which is generated through interactions between T cells involving their antigen-specific receptors. In this model, derepression is achieved by competition of antigen with the interactions among T cells through which suppression is exerted. To test our model, we analyze in this paper how much of the immune response to keyhole limpet hemocyanin (KLH) in draining lymph nodes is accounted for by an increase in the numbers of KLH-reactive T cells or by their derepression. To this end, we immunize mice subcutaneously with KLH in CFA. For a period of 14 days after immunization draining lymph nodes are removed, and the frequencies of KLH-reactive proliferating T cells and of KLH-reactive helper T cells determined. We find that proliferating T cells increase 5 to 8 fold in frequency from day 1 to 4 after immunization (approximately 1/30,000 to approximately 1/5000) and no evidence for suppression of these T cells in the non-immune situation can be obtained. In contrast, T helper cells are strongly suppressed in non-immune lymph nodes and become derepressed suddenly between days 3 and 4 following immunization. From day 0 to day 3 T helper cell frequencies are in the order of 1/14,000-1/38,000, then increase suddenly approximately 3-6 fold within 1 day from 1/16,000-1/8,000 to 1/3,000-1/5,000 with no further change until day 14. Thus, helper T cell immunity in draining lymph nodes appears to be generated by a combination of increased frequencies of specific T cells with their release from suppression. In addition, we have reasons to suspect that we overestimate the increase in T cell frequencies. We therefore think that derepression is a major factor in the response of T helper cells to antigen.