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
Murine kidney transplant surgeries were performed from C3H (H-2D^k) to B6 (H-2D^b), in WT and various LAG3 knockout (KO) strains and mice were monitored for rejection. Grafts were characterized by histology. Assessment of immune responses was performed by flow cytometry of the immune cells in the spleen and infiltrating the allograft, ELISPOT assay, and the levels of donor specific antibody (DSA) in the serum.
Compared to WT animals, naïve B6.LAG3^-/- mice have increased splenic cellularity and higher frequencies of CD44hi memory T cells, CXCR5hi follicular T cells, and B220⁺CD138⁺ plasma cells yet do not develop spontaneous autoimmunity. Furthermore, naïve B6.LAG3^-/- mice have increased frequencies of memory T cells against H-2D^d, H-2D^s, H-2D^q and H-2D^k alloantigens and elevated antibody titers against H2-D^d, H2-D^k, I-A^d and I-A^k. C3H kidney allografts were transplanted into B6.WT or B6.LAG3^-/- recipients after bilateral nephrectomy. Whereas 4/4 WT recipients accepted C3H allografts beyond 60 d, recipient LAG3 deficiency led to rapid allograft rejection (MST of  14 d, n=5) with serum creatinine levels of 0.1 and 1.35 mg/dl respectively 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 16 d), while B cell depletion significantly extended C3H kidney allograft survival (MST of >30 d), suggesting the predominant role of alloantibody rather than T cell mediated rejection in these mice. However, using conditional knockout recipients using CD4Cre or CD19Cre mice we discovered that deficiency of LAG3 on both cells is necessary to mediate rejection.
These findings demonstrate that LAG3 regulates both T and B cell functions in response to kidney allografts, and as such is an attractive therapeutic target for the prevention of AMR.