*Purpose: The functions of coinhibitory receptor lymphocyte activation gene-3 (LAG3) in T cells are well studied, however its role in humoral immune responses remains poorly characterized. The goal of this study was to test the role of recipient LAG3 in a mouse model of renal allograft rejection.

*Methods: Murine kidney transplant surgeries were performed from C3H (H-2D^k) donors to B6 WT, LAG3^-/-, CD4CreLAG3^fl/fl & CD19CreLAG3^fl/fl recipients (H-2D^b) after bilateral nephrectomy. Grafts were evaluated by immunistochemistry. WT recipients were treated with anti-mCD20 or anti-CD8 mAbs to deplete B cells or CD8 T cells, respectively. Immune responses were assessed by flow cytometry, ELISPOT assay, and serum levels of MHC-I and MHC-II-reactive IgG donor specific antibody (DSA) were determined by ELISA.

*Results: Compared to WT animals, naïve B6.LAG3^-/- mice have elevated numbers of CD44^hi memory T cells, CXCR5^hi follicular T cells, and B220⁺CD138⁺ plasma cells, and increased frequencies of memory T cells reactive to H-2D^d, H-2D^s, H-2D^q and H-2D^k alloantigens. All C3H kidney allografts survived for > 60d (n=5) in WT recipients, whereas recipient LAG3 deficiency led to rapid allograft rejection (MST of 14d, n=5) and elevated serum creatinine levels at d14 posttransplant. Graft histology at rejection revealed minimal T cell infiltration, diffuse C4d staining, atrophic peritubular capillaries, endothelial swelling and edema characteristic of antibody mediated rejection (AMR). Compared to WT, LAG3^-/- recipients had elevated frequencies of anti-donor IFNγ producing T cells and increased levels of DSA against MHC-I and MHC-II. Recipient CD8 T cell depletion did not alter rejection kinetics in LAG3^-/- recipients (MST of 16d), while B cell depletion significantly extended C3H kidney allograft survival (MST of >30d), suggesting the predominant role of alloantibody rather than T cell mediated rejection in these mice. However, neither T nor B cell conditional knockout recipients rejected the allograft demonstrating LAG3 expression on both cell types is necessary to mediate rejection.

*Conclusions: These findings demonstrate that LAG3 regulates both T and B cell functions in response to kidney allografts, and is an attractive therapeutic target for the prevention of AMR.

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