*Purpose: Laminin α5 (Lama5) regulates CD4 T cell migration and function. We hypothesized that stromal Lama5 regulates the structure of the lymph node (LN) microenvironment to channel T cell distribution and alloimmunity.

*Methods: In stromal cell Lama5 conditional knock out, Pdgfrb-Cre^+/-xLama5^fl/fl, (KO) mice, Treg distribution, chemokines, and cell adhesion molecules were analyzed by immunohistochemistry (IHC) and qRT-PCR. C57BL/6 wild type (WT) and Lama5 KO mice received cardiac transplants from BALB/c donors. Recipients received low dose anti-CD40L mAb or tacrolimus immunosuppression to assess graft survival and histology and T cell responses. Recipients were also treated with blocking mAbs to the Lama5 receptors α6 integrin or α-dystroglycan (αDG).

*Results: There were greater numbers of Treg in LN T cell zones of KO compared to WT. Greater numbers of dendritic cells (DC) marked by CD11c or PDCA1 were also observed in the KO cortical ridge (CR) and around high endothelial venules (HEV). Chemokine transcripts for CCL19, CCL21 and CXCL12 were increased in KO fibroblastic reticular cells (FRC), while CXCL12 and CCL21 protein expression were increased in the CR and around HEV, respectively. Transcripts for ICAM-1 and VCAM-1 but not MAdCAM-1 were increased in KO FRC. VCAM-1 protein was increased in KO CR and HEV compared with WT. Tacrolimus treated KO recipients had significantly longer allograft survival (mean survival time (MST) 89 days) than WT (MST 27.5 days, p<.002). KO recipients receiving a single dose of anti-CD40L displayed a trend for increased survival (MST 155 vs 91 days, p=0.07). Blocking with anti-α6 integrin or anti-αDG mAbs had additive effects to genetic depletion of Lama5. Compared to isotype controls, anti-αDG plus anti-CD40L treated recipients had prolonged graft survival (MST 65 vs 154 days, p<0.002); similarly, anti-α6 integrin plus anti-CD40L prolonged survival (MST 61 vs 159 days, p<0.002). With tacrolimus, graft survival was prolonged with anti-α6 integrin (MST from 28.5 to 67.5 days, P<0.005) and anti-αDG (MST from 19 to 63.5 days, p<0.001).

*Conclusions: Depletion of Lama5 upregulated chemokines, adhesion molecules, DC and Treg in the T cell zones, CR and around HEV. Lama5 deficient mice have a tolerant LN niche that facilitates Treg entry into LN. Depleting LN stromal Lama5 and systemic blockade of its receptors worked additively to promote allograft acceptance. These results suggest targeting Lama5 is a new pathway to promote tolerance.

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