CD4 and infection: Once hematopoietic reconstitution was established, the chimeras were infected with aerosolized M. tuberculosis and analyzed four weeks later, during the peak of the T cell response to M. tuberculosis. After infection, WT CD4+ and CD8+ T cells, but not B cells, were overrepresented in blood relative to IL-21R−/− cells (Fig. 3a–d).