MAPCs do not stimulate T-cell responses and are susceptible to NK-cell–mediated lysis. (A) MAPCs are low/negative for MHC class I and class II, CD80 (B7-1), CD86 (B7-2), ICAM-1, and CD40. Upon 24 hours stimulation with 1000 IU/mL IFN-γ, MHC class I and ICAM-1 expression was up-regulated, whereas the expression of MHC class II, CD80, CD86, and CD40 remained low. Representative histograms of experiments performed 3 times are shown. (dashed line) Irrelevant antibody. (continuous line) Antibody to antigen indicated in each panel. (B) 1 × 105 BALB/c CD4+ T cells/well or (C) 1 × 105 BALB/c CD4+ plus CD8+ T cells/well and irradiated, untreated B6 MAPCs or irradiated MAPCs (104/well) that were pretreated with 1000 IU IFN-γ/mL for 48 hours were mixed in T-cell proliferation assays. In some wells, T cells were cultured alone or with irradiated T-cell–depleted B6 splenocytes (105/well). T-cell proliferation was measured by 3H-thymidine uptake on day 5 and is expressed as mean ± SEM. (D) 105 BALB/c CD4+ plus CD8+ T cells per well were cultured with the indicated number of allogeneic splenic stimulators, MAPCs, or MAPC DLs per well. Mean background counts for 105 irradiated B6 splenocytes, MAPCs, or MAPC DLs were 37, 362, and 274 cpm, respectively (data not shown). (E) After a 96-hour primary MLR culture with irradiated B6 splenic stimulators or MAPCs, 105 BALB/c CD4+ plus CD8+ T-cell responders/well were restimulated with 4 × 104 freshly prepared B6 splenic stimulators, MAPCs, or MAPC DLs per well. Mean background counts of 4 × 104 irradiated B6 splenocytes, MAPCs, or MAPC DLs were 82, 434, and 507 cpm, respectively (data not shown). (F) To determine whether MAPCs are susceptible targets for NK-cell–mediated killing, splenocytes from poly I:C–treated B6 mice were mixed with Yac-1 cells or with MAPCs in a chromium release assay. Effector–target cell ratios showed that MAPCs are a target of NK lysis in vitro.
