*Purpose: Microparticles (MPs) carrying peptide antigen with immune signals can be directly delivered to lymph nodes (LNs) via intra-LN injections to shape both pro-inflammatory or anti-inflammatory immune responses. We hypothesized that since dendritic cell (DC) antigen presentation is important for antigen specific generation of tolerogenic immune responses, that MP loaded with alloantigen and rapamycin will regulate DC antigen presentation.

*Methods: MP were composed of poly (lactide ­co-­ga lactide). Rapamycin (Rapa) and/or donor class II alloantigen (I­E^d peptide (Ea)) were incorporated into the MP during assembly. C57BL/6 mice underwent subcutaneous injection of tracer dye Evans Blue at the tail base with subsequent visualization and injection of inguinal LNs with empty, Ea-, Rapa-, or Rapa/Ea-MPs. Animals were euthanized 1-10 days post MP injection and injected, non-injected, draining, and non-draining LNs harvested. Alloantigen presentation was visualized with YA-e monoclonal antibody which recognizes donor Ea peptide presented by recipient MHC II DC.

*Results: Naive C57BL/6 mice display minimal expression of YA-e staining at baseline without previous exposure to alloantigen or MPs. YA-e staining for DC antigen presentation increased in the inguinal LNs directly injected with Rapa/Ea-MPs, starting at the day 1 post-injection, with peak antigen presentation occurring at days 5 and 7. The YA-e+ DC surrounded the HEV and cortical ridge, the sites where DC induce Foxp3+ regulatory T cells. Increased YA-e staining also occurred in non-injected draining axillary LNs, with no differences in the non-draining popliteal LN. There were no differences in CD11c expression in the HEV or cortical ridge of injected or uninjected LNs.

*Conclusions: MPs modify the local immune microenvironment by inducing robust DC antigen presentation needed for the initiation of transplant tolerance. DC antigen presentation in the LN microdomain is required for regulatory T cell induction. DC antigen presentation is also present in both injected and draining LNs, suggesting rapid migration of DC from the injected LN to the periphery. This suggests that local LN manipulation is rapidly translated into systemic suppression. These findings demonstrate the ability of MPs to manipulate the local LN milieu, resulting in systemic antigen presentation beneficial for suppression and tolerance.

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