CD4 and lymphopenia: In summary, following autologous HSPC transplant: (1) thymic export and downregulation of CCR5 drive a modest expansion of CD4+CCR5- T cells, whereas (2) rapid lymphopenia-induced proliferation after TBI is the main driver for CD4+CCR5+ and CD8+ T cell expansion, which are derived from both the transplanted HSPC product and residual endogenous cells that persisted through the myeloablative conditioning regimen.