As shown in Figure 1A, donor CD4+ T cells, as expected, exhibited only ~2–3 rounds of slow rate of cell division (i.e., un-gated CTV+ cells), referred to as lymphopenia-induced homeostatic proliferation (LIP) that is known to be dependent on TCR interaction with self-ligands and cytokine IL-7 (3, 7). Here, CD4 is linked to lymphopenia.