*Purpose: Monocytes differentiate into macrophage subsets that orchestrate tissue repair in response to damage signals and local cytokines. After heart transplant (Tx), graft IL-33 acts as a reparative/regulatory damage-associated molecular pattern (rDAMP) that metabolically reprograms macrophages to enable their differentiation into reparative subsets that protects against chronic Tx rejection. Yet, innate allorecognition of MHCI (H2-D^d) by paired immunoglobulin-like receptor (PIR)-A3 on monocytes generates pro-inflammatory monocyte-derived dendritic cells that promotes BALB/c kidney and heart Tx rejection. PIR-A responses are thought to be negatively regulated by co-expressed PIR-B binding to self MHCI. How PIRs shape macrophage differentiation from monocytes after Tx is poorly understood. Thus, we hypothesize that recognition of self versus allogeneic MHCI by PIRs modulates rDAMP-mediated macrophage differentiation into reparative macrophages following Tx.

*Methods: Macrophage differentiation in response to donor materials was tested by administration of 20×10⁶ irradiated BALB/c allogeneic (H-2^d; Allo) or B6 syngeneic (H-2^b; Syn) splenocytes intraperitoneally and/or with recombinant IL-33. Flow cytometry of peritoneal leukocytes was utilized to assess macrophage differentiation in wildtype (WT), Rag2^-/-γc^-/-, Pira^-/- and Pirb^-/- B6 mice.

*Results: Alloantigen transformed local macrophages by 3 days post donor cell delivery. WT and Rag2^-/-γc^-/- mice had profound increases in monocyte-derived inflammatory Ly6c^hi CD86^hi macrophages compared to syn-treated mice (Fig. 1a,b). This shift was at the expense of CD206⁺ CD301⁺ macrophages, which dominated after PBS or syn exposure (Fig. 1c). Reparative macrophage loss was rescued by co-administering IL-33. Interestingly, Pirb^-/- mice had greatly reduced C206⁺ CD301⁺ reparative macrophages and decreased expression of the IL-33 receptor, Stimulation-2 (ST2).

*Conclusions: These data indicate allorecognition by monocytes or macrophages can disrupt the typical generation of reparative macrophages in response to injured self. We also show that monocyte-derived macrophage PIR-B recognition of self-MHCI may trigger ST2 expression needed for IL-33-stimulated differentiation into reparative macrophages. These new data provide important insights into how innate allorecognition and repair pathways intersect to direct the differentiation of reparative or inflammatory macrophages after Tx.

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