Survival levels of NSG–BLT mice were 51·1% (24 of 47 mice surviving) by 28 weeks post-implant compared to 86·7% (14 of 16 mice surviving) survival of irradiated-only control NSG mice that did not receive human tissues. We next evaluated if the number of CD34+ HSC injected influenced the incidence of xeno-GVHD in NSG–BLT mice, as indicated by the time of death. NSG mice that were irradiated and then implanted with human fetal thymic and liver tissues and injected with the indicated number of CD34+ HSC were monitored

for Y-27632 in vitro survival over 200 days (Supporting information, Fig. S8a). The data show that there is no correlation between the number of CD34+ HSC injected and the incidence of xeno-GVHD. In addition, we found no correlation between the percentages of CD3+ T cells in the peripheral blood of NSG–BLT mice at 12 weeks and incidence of xeno-GVHD (Supporting information, Fig. S8b). We also found no differences in the incidence of xeno-GVHD between NSG–BLT mice implanted with female and male tissues (Supporting information, Fig. S8c). The decrease in naive phenotype human CD4 and CD8 T cells in older NSG–BLT mice (Fig. 5) suggests that these T cells are being activated and mediating a xenogeneic GVHD. We hypothesized that the development of xeno-GVHD in NSG–BLT mice might result from a lack of negative selection against murine antigens in the human thymus or by a lack of peripheral regulation. Our previous studies showed that the xenogeneic

GVHD occurring after the injection of human RO4929097 supplier PBMC into NSG mice is mediated by T cell recognition of murine MHC (H2) classes I and II [55, 56]. To test if H2-reactive human T cells escape negative selection and contribute to the mortality of older NSG–BLT mice, NSG mice lacking the expression of murine MHC class I [NSG-(KbDb)null] or class II (NSG-Abo), were used to engraft fetal thymic and liver tissues. NSG-(KbDb)null

and NSG-Abo BLT mice did not have increased overall survival compared to standard NSG–BLT mice (Fig. 6a). Unexpectedly, the survival of engrafted NSG-(KbDb)null mice was reduced significantly compared to NSG–BLT mice (P < 0·001, Fig. 6a). Human cell chimerism Aldol condensation (huCD45+ cells) was compared in the blood at 12, 16 and 20 weeks in NSG mice, NSG-(KbDb)null and NSG-Abo mice (Fig. 6b). Human CD45+ cell chimerism was comparable in the three NSG strains. Together, these data suggest that elimination of either murine class I or murine class II is not sufficient to overcome the mortality of older NSG–BLT mice. We next compared the engraftment and survival of NSG–BLT mice to BLT mice that were co-implanted under the renal capsule with 1 mm3 fragment of fetal mouse liver (fml) and human thymic tissue, in an attempt to enhance negative selection against murine antigens. Co-implant of fml did not increase the proportion of mouse cells (murine CD45+ staining) detected within human thymic organoid (Fig. 6c). Overall engraftment in the blood of both sets of mice was similar at 12 weeks after implant (Fig.