CD38 and malaria: Furthermore, a malaria-specific cell-mediated immune response to the GAP vaccine was identified in two of the three subjects (subject 5 and subject 6; not tested in subject 7) administered 3× 106 parasites, including increased expression of activation markers PD-1 and ICOS on Th1-like Tfh cells (Fig. 3B) and in the expression of activation markers CD38 and HLA-DR on Th1 cells (Fig. 3C) on day 28.