*Purpose: Laminin α5 (Lama5) is associated with immunity and regulates T cell entry and responses within the lymph node (LN). We hypothesized that stromal Lama5 depletion would impair T cell activation in the LN, thereby suppressing allograft rejection.

*Methods: A stromal Lama5 conditional KO (KO^fl) mouse model Pdgfrb-Cre^+/-xLama5^fl/fl was established. Wild type (WT) and KO^fl mice on a C57BL/6 background received BALB/c donor-specific splenocytes (DST), or DST plus anti-CD40L mAb for immunity and tolerance induction, respectively. T cell receptor transgenic CD4+ TEα cells recognizing donor I-E^d presented by recipient I-A^b (Eα antigen) were transferred into recipients; and TEα cell migration, distribution, activation, and differentiation were measured. BALB/c donor hearts were transplanted to WT and KO^fl mice, and graft survival was measured.

*Results: More antigen specific TEα cells migrated into KO^fl LN compared to WT under both immune and tolerance conditions. Greater TEα cell numbers accumulated in the cortical ridge and around the high endothelial venules (HEV) in KO^fl compared with WT controls. TEα cell activation to CD44^hiCD69⁺ cells were lower in KO^fl LN and spleen compared to WT, indicating that depletion of Lama5 prevented T cell activation. Under tolerance conditions, activation induced cell death of TEα cells in LN and spleen was lower in KO^fl compared to WT, showing that inhibition of T cell activation preserved antigen specific T cells. Under both tolerance and immune conditions, TEα cell differentiation toward Foxp3+ Treg and IL-17+ Th17 was altered, so that the Foxp3:Th17 ratio was higher in KO^fl LN than in WT. Low dose tacrolimus (2 mg/kg/d) treated KO^fl cardiac recipients had significantly prolonged allograft survival (mean survival time (MST) 89 days vs 27.5 days in WT, p<.002). Similarly, low dose anti-CD40L mAb treated KO^fl recipients also had prolonged allograft survival (MST 155 vs 91 days, p=0.07).

*Conclusions: Depleting stromal Lama5 promoted antigen-specific CD4 T cell migration to the LN cortical ridge, suppressed their activation, and channeled their differentiation from inflammatory to suppressive regulatory phenotypes. Depleting stromal Lama5 also promoted Treg induction and inhibited Th17 differentiation, creating a tolerogenic niche to enhance cardiac allograft survival. Targeting Lama5 on stromal cells of LN could lay the ground for establishing innovative tolerogenic approaches to improve cardiac graft survival.

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