Expression of CD45RB and CD27 identifies subsets of CD4⁺ memory T cells with different capacities to induce B cell differentiation

The capacity of four subsets of CD4⁺ memory T cells, defined by expression of CD45RB and CD27, to provide help for B cells was examined. Larger amounts of Ig were induced by CD45RB(dim)CD27^- cells compared with the CD45RB(dim)CD27⁺ population, whereas CD45RB(bright)CD27⁺ or CD27^- cells were poor inducers of Ig synthesis. Mitomycin C treatment, which prevents suppressive activity, markedly enhanced Ig production supported by each subset except for CD45RB(bright)CD27^- cells. Mitomycin C treated CD45RB(dim) cells remained the most efficient inducers of Ig production, but no difference was detected between CD27⁺ and CD27^- cells. The subsets also differed in their ability to proliferate and secrete cytokines, but these differences did not explain variations in the capacity to provide help for B cells. Both CD27^- subsets exhibited decreased proliferation and uniquely secreted IL-4, with the CD45RB(dim)CD27^- subset producing the greatest quantities of IL-4. No differences in IL-2 and IFN-γ, production were found. IL-10 secretion increased with the acquisition of the CD45RB(dim) phenotype and, within the CD45RB(dim) cells, with the loss of CD27. Staining for cytoplasmic cytokines indicated that individual populations of CD27^-CD4⁺ helper T cells produced either IL-4 or IFN-γ, whereas more than half of the IL-4 producers also synthesized IL-2. Finally, the different abilities of CD4⁺ memory T cell subsets to support B cell differentiation did not relate to variations in the expression of CD40 ligand. These results indicate that within the CD4⁺ memory T cell population an increase of helper activity associates with the shift from a CD45RB(bright) to a CD45RB(dim) phenotype. Within the CD45RB(dim) subset, the loss of CD27 is associated with a reduction of suppressive activity.