CD4 and lymphopenia: For this, either a small or large number of CD4+ CD8− thymocytes (that are Ly6C–; 25 × 106 or 2.5 × 106, respectively) were injected i.v. into irradiated B6 hosts (Figure 2B, top), in which the donor cells undergo lymphopenia-induced homeostatic proliferation (LIP) in a manner dependent on TCR self-interaction.