*Purpose: Transplantation is the primary treatment for patients with end-stage organ failure. However, long-term outcomes are limited by immunosuppressant toxicities and chronic graft rejection. Identifying factors involved in regulatory T cell (Treg, CD4⁺ CD25⁺ FOXP3⁺) development and commitment is required for realization of their therapeutic potential in solid organ transplantation. Fibrinogen-like protein 2 (FGL2)-deficient Tregs are functionally defective, so we tested the hypothesis that FGL2 is required for normal thymic Treg development.

*Methods: We first generated reciprocal FGL2 bone marrow chimeras by reconstituting B6 CD45.1 or CD45.2 female mice with bone marrow stem cells from either FGL2^-/- (KO) or FGL2^+/+ (WT) mice, producing WT->KO and KO->WT chimeras and corresponding KO->KO and WT->WT controls (Fig.1A). Reconstitution was monitored by monthly tail bleed. Once the reconstitution reached ≥90%, plasma, spleens, and thymi were collected. We analyzed the plasma using ELISA for FGL2 and thymocytes with flow cytometry.

*Results: WT->KO chimeras had a faster rate of reconstitution than WT->WT chimeras. Analysis of thymocyte developmental stages revealed no significant differences between the groups. Thymi from KO->WT chimeras had a significantly lower number of donor-derived Tregs, whereas thymic Tregs were present in higher numbers in KO->KO and WT->KO chimeras (Fig.1B). PD-L1 expression differed dramatically among the groups, with KO->WT and WT->KO chimeras exhibiting an elevated percentage and absolute number of PD-L1⁺ cells compared with the other chimeras (Fig.1C-D). No differences were observed in GITR, TIGIT, LAG3, or PD-1 expression on thymic Tregs from the different groups. Lastly, there were no significant differences in FGL2 plasma levels between the different chimeras.

*Conclusions: Our results indicate that circulating FGL2 is derived from a radioresistant source and influences the rate of T cell reconstitution. Both radiosensitive and radioresistant sources of FGL2 affect thymic Treg expression of PD-L1, an important molecule for Treg function. In future work we will examine how FGL2 modulates Treg maturation, function and phenotype in the steady state and upon activation.

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