| Abstract: | Although studies have identified factors which affect germinal centre cell proliferation in vitro, their relative contributions in vivo remain largely undetermined. In this study, the proliferative rate of germinal centre cells was measured in sheep red blood cell-immunized C3H/HeN mice exposed to variously timed doses of cyclosporin A. Germinal centre (GC) cell proliferation was measured by a stathmokinetic technique to determine GC cell birth rates at specific time points after immunization. Changes in total GC volume were determined by morphometry in order to assess actual growth and regression of the GC cell population. An estimate of the absolute rate of GC cell proliferation was derived from these two values. Following exposure to antigen, there was an initial inhibition of proliferation within pre-existing germinal centres, followed by a rapid rise, then a sustained phase of increased GC cell proliferation. By comparing the effects of the different cyclosporin A treatment regimes, it was possible to deduce that the initial inhibition of proliferation was mediated by a T-cell-derived cytokine, as was the final sustained phase of the proliferative response. The intervening rise in GC cell proliferation, however, was attributable to a contact-dependent signalling mechanism. |
