Real-Time In Vivo Biodistribution of Multipotent Adult Progenitor Cells (MAPC): Role of the Immune System in MAPC Resistance in Non-Transplanted and Bone Marrow Transplanted Mice.
Authors: Tolar Jakub , Bell Scott , McElmurry Ron , Xia Lily , McIvor R. Scott , Yant Stephen R. , Kay Mark A. , Contag Christopher H. , Verfaillie Catherine M. , Blazar Bruce R. 
Published: April 2020

Abstract
MAPC are non-hematopoietic stem cells derived from adult BM with the potential for a wide differentiation pattern in vitro and in vivo. MAPCs are MHC class I and thus may be a target of natural killer (NK) cell mediated elimination in the syngeneic setting. To determine whether MAPC are susceptible targets for NK mediated killing, splenocytes from poly I:C (an inducer of NK activity) treated C57BL/6 mice were mixed with Yac-1 (H2a; a NK sensitive target) or MAPC (from C57BL/6J-rosa26) in a chromium release assay. Effector:target ratios indicated that MAPC were susceptible to NK lysis albeit less so than Yac-1 cells. To assess in vivo immune responses to MAPC, we infused MAPC into mice with various degrees of T-, B-, and NK- cell immune competence. To follow biodistribution of MAPC in live animals with whole body imaging (WBI), we labeled MAPC with red fluorescent protein DsRed2 and luciferase, using Sleeping Beauty transposons. MAPC (106) were co-nucleofected (Amaxa) with 5mcg of each pT/CAGGS-DsRed2 and pT/CAGGS-Luciferase and an SB transposase-encoding plasmid (p/CMV-HSB2) at a 1:50 ratio. Selected double transgenic MAPC (MAPC DL) clones were euploid, and maintained their characteristic trilineage differentiation. MAPC DL (106) were injected IV into cohorts (n=5–6) of adult C57BL/6 (B6), Rag2−/− (T- and B-cell deficient) and B6 Rag2/IL-2Rgc (T-, B- and NK deficient mice). Additional cohorts of B6 and Rag2−/− were given anti-NK1.1 mAb 2x/wk to deplete NK cells. In B6 mice, MAPC DL were detected on d4 but not d14 or d30. In Rag2−/− mice, MAPC DL were detected throughout the 30d period. NK depletion did not substantially increase MAPC DL number in B6 mice. However, in Rag2/IL-2Rgc mice MAPC DL were persistent and in 50% of mice they increased in number from d4‡d30. Post-mortem analysis revealed MAPC DL cells in all but B6 wild type mice: Rag2/IL-2Rgc ≥ Rag2−/− with NK depletion&gt;&gt; Rag2−/−. These data suggest that endogenous NK cells and T cells resist MAPC DL. Interestingly, in vitro studies indicate that MAPCs suppress an allogeneic mixed lymphocyte reaction (MLR) culture. Therefore, the T cell resistance to MAPC may be due to an immune response generated to the multiple foreign reporter proteins expressed by these cells. Since MAPCs may be useful as cellular therapies for the treatment of regimen-related toxicity, studies were performed in which B10.BR mice were lethally irradiated (TBI) and given B6 BM ± MAPC DL (106). MAPC DL were seen in the chest, abdomen, face, and paws on d4, d7, d10 and d28 at high numbers suggesting that TBI conditioning overcomes both NK and T cell mediated resistance resuting in a widespread homing/migration of MAPC. These data are the first to illustrate the immune responses to MAPCs and indicate that TBI conditioning may be advantageous in the long-term survival and widespread homing of MAPCs.