*Purpose: Solid-organ transplant recipients are placed on long-term immunosuppression, preventing graft rejection but increasing the risk of infection. Pediatric transplant recipients are at increased risk for infections. We seek to identify biomarkers to predict risk of infection and discover potential causes of the increased susceptibility to infection for pediatric kidney and liver allograft recipients.

*Methods: In a single-center registry study, we analyzed 66 kidney and 18 liver pediatric transplant recipients. Immunologic labs including lymphocyte subsets, immunoglobulins, and vaccine titers were collected at 6-month intervals as part of standard institutional solid-organ transplant care. Number of infections and interval health history were recorded in corresponding 6-month intervals along with immunosuppressive regimen and dosage. The relation between immune parameters and number of infections were determined via linear mixed regression model of all consecutive visits while association between individual medications and immune parameters was analyzed via a paired t-test.

*Results: For every 1% increase in the ratio of naïve to memory B cells, there was an increase in number of total infections by 0.04 (β=0.04, p<0.0001). A higher naïve B-cell percentage (β=0.03, p=0.001) and a lower memory B cell percentage (β=-0.03, p=0.002) were also associated with increased total infections. For every increase by 1% of class-switched memory B cells (CD27+) resulted in a decrease of 0.04 in total infections (β=-0.04, p=0.003) and non-switched memory B cells in a decrease of 0.07 in total infections (β=-0.07, p=0.011) across visits. No significant changes were seen in Ig levels, T-cell populations, or vaccine titers. Mycophenolate mofetil (MMF) therapy was associated with increased naïve B cell percent (β=12.4, p<0.001), decreased CD27 B cell percent (β=-10.3, p<0.001), and increased naïve:memory B cell ratio (β=4.5, p=0.009). Azathioprine was associated with the opposite trends.

*Conclusions: Disruption of the naïve:memory B cell ratio could be an indicator of increased infection risk in solid-organ transplant recipients. The changes seen with MMF were associated with increased infections and indicate MMF as a potential culprit for increased infection risk. The ratio of naïve:memory B cells has potential as a biomarker to predict risk of infection in immunosuppressed solid-organ transplant recipients especially in those on MMF based immunosuppression.

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