Immunoglobulin production induced by CD57+ GC-derived helper T cells in vitro requires addition of exogenous IL-2

Germinal centers (GC) are well-defined areas in lymphoid organs were B cells proliferate and differentiate in response to T-cell-dependent antigens. The GC comprises B cells, follicular dendritic cells, tangible body macrophages, and a low number of CD4+ T cells. A large portion of these T cells expresses CD57. We have examined the ability of the CD4+ CD57+ GC T cells to become activated and to take part in B cell activation processes. These T cells coexpress CD45RO, CD69, CD28, and upon mitogenic stimulation CD25. The cell population was found neither to contain nor to be able to produce any specific mRNA for IL-2, IL-4, and IFN-gamma upon activation. Levels of mRNA encoding CD40 ligand was also undetectable under similar conditions. Furthermore, in contrast to ordinary CD4+ T cells, this population expressing CD57 was unable to induce B cells to Ig production in the presence of pokeweed mitogen or SEA unless IL-2 was added to the cultures. However, despite their apparent lack of function CD4+ CD57+ GC T cells were found to rescue GC B cells from cell death in vitro to the same extent as CD4+ CD57+ Th cells. The phenotypical and functional differences found between these Th cells and regular Th-cells suggest that they either represent a T cell subset with distinct properties within the GC yet to be determined or that they represent T cells, late in the immune response, having lost most of their original functions and capabilities.