Genetic and functional evidence that human B2M triggers IL-7 production by human TECs and augments thymocyte counts. (A) Western blot of B2M developed by Enhanced ChemiLuminescence shows the absence of the B2M protein in DCs in which the human B2M gene was silenced with siRNA vs its presence in control DCs (untreated or electroporated or treated with scramble siRNA). (B) Supernatants from cocultures of human alloreactive NK cells and B2M-silenced DCs failed to increase human thymocyte cellularity (see the lane labeled “B2M siRNA”). (C) Increase in human thymocyte cellularity upon addition of human recombinant B2M in the TEC/thymocyte culture system. (D) Human alloreactive or nonalloreactive NK cell clones were cocultured with human DCs. NK/DC coculture supernatants were added to human TECs. Unlike supernatants from nonalloreactive (auto) NK/DC cocultures, the supernatants from alloreactive NK/DC (allo) cocultures promoted IL-7 production by TECs. (E) When thymocytes were cultured with TECs, the addition of an anti–IL-7 Ab prevented the increase in thymocyte counts mediated by an alloreactive NK/DC coculture supernatant. Bars: mean ± standard deviation of at least 3 independent experiments; statistics were performed with a Student test (GraphPad Prism 5). ∗P < .05; ∗∗P < .01. siRNA, short interfering-RNA.