Standard Drug Immunosuppression Eliminates Peripheral Transitional B Cells but Does Not Create a Distinct Genetic Signature in Non-Human Primates

Purpose Operationally tolerant renal transplant recipients exhibit an increased number of transitional B cells and an upregulation of selected B-cell associated genes in the periphery compared to patients on chronic immunosuppression. Whether this signature is associated with tolerance induction or immunosuppressive drug therapy alone is unclear. To test this, we investigated the effects of triple drug immunosuppression on the frequency of B cell subsets and gene expression in the peripheral blood of cynomolgus macaques . Methods Comparisons were made between four groups: 1) naive monkeys treated daily for 150 days with Tacrolimus (0.1 mg/kg), Mycophenolate mofetil 200mg, and methylprednisolone 40mg tapered to 1mg over 2 weeks (ISA n=4), 2) recipients of heart + kidney, heart + thymus, or heart alone allografts on the same immunosuppression (SI n=3), 3) untreated naive controls (UC n=13), and 4) recipients tolerant of allografts (TOL n=4). Cryopreserved PBMCs were stained for flow cytometry analysis with the following antibodies: CD3, CD20, CD38, CD27, IgD, IgM, CD8, and eFluor viability die. RNA was isolated and sequenced from cryopreserved PBMC, and differential expression at the gene level was analyzed with DESeq2. Results After one month of treatment, the frequency of peripheral transitional B cells (CD38 hi CD27-IgD+) in ISA animals had decreased to less than 10% baseline values lasting the duration of treatment. TOL animals appeared to have a similar phenotype as UC animals, while ISA animals had a significant decrease in transitional B cells compared to both UC and TOL animals. TOL animals did not exhibit an enhanced expression of B cell specific genes compared with SI animals, and gene expression did not differ significantly from UCs. The ISA group did not show significant differential expression compared to TOL and UC animals. Conclusion The decrease in transitional B cells in NHPs after 3 months of immunosuppression suggests that the apparent increase of transitional B cells in tolerant patients may be due to the withdrawal of immunosuppressive drugs rather than an active tolerance process. Gene expression data suggest that a B cell gene signature may not be a reliable predictor of tolerance in NHPs, but also indicates that expression with immunosuppression alone doesn't differ greatly from untreated controls.