Introduction: The adaptive immune system is known to play a central role in solid organ transplant tolerance and rejection. The aim of this study was to characterize populations of T and B cell subsets in pediatric liver transplant (LTx) recipients via immunophenotyping. We hypothesized that such analysis would identify a unique cellular immune profile of tolerance in pediatric LTx recipients. Methods: Twenty pediatric LTx recipients were classified into 3 clinical phenotypes: tolerant (TOL, n=6) with normal liver function off immunosuppression for ≥2 years; non-tolerant (NON-TOL, n=6) with a history of ≥1 rejection episode ≥1 year post-LTx on double or triple immunosuppression; stable (STA, n=8) on tacrolimus monotherapy with normal liver function and no rejection episodes ≥1 year post-LTx. PBMCs were separated from peripheral blood specimens collected at routine clinic visits. Immunophenotyping was performed with multi-color monoclonal antibody panels. Cell fluorescence was acquired on an LSR FortessaTM cell analyzer. Population measurements were determined using FCS Express V4 analysis software. Non-transplanted, healthy young-adults (n=3) were used as controls to establish and validate flow panels. Statistical analyses included fisher's exact, Kruskal-wallis, and t-tests. Results are reported as mean ± SD. Results: As shown in the Table, TOL patients tended to be younger at the time of LTx and older at the time of sample collection. All TOL patients were males. As also shown in the table, T and B cell populations varied across groups. TOL patients had a significantly greater proportion of naïve CD4+RA+ T cells and smaller populations of central memory CD4+CD197+RA- T cells compared to NON-TOL and STA. TOL patients had a trend towards lower proportions of memory B cells (CD19+CD27+) and higher proportions of transitional (CD27CD-38++IgD+) and unswitched memory (CD27+IgD+IgM+) B cells compared to NON-TOL patients. TOL patients also had increased naïve CD4+CD197+RA+ T cells, CD4+CD25+CD127l0 regulatory T cells, and inducible CD4+CD25+CD127l0RA- T cells (data not shown).Table: [Results for TOL, NON-TOL, and STA LTx recipients]Conclusion: This study represents the most comprehensive analysis of T and B cell immunophenotypes in pediatric LTx recipients to date. We have shown that TOL patients have a unique adaptive immune system cellular profile characterized by smaller populations of central memory T cells and larger populations of naïve and regulatory T cells as well as transitional and unswitched memory B cells. This may lead to fewer effector T cells and increased production of regulatory cytokines, ultimately creating an overall tolerogenic environment. We are currently immunophenotyping a larger cohort of patients to further delineate the unique features of tolerance and discriminate them from the effects of immunosuppression.