Impaired thymic function and CD4+ T lymphopenia, but not mannose-binding lectin deficiency, are risk factors for Pneumocystis jirovecii pneumonia in kidney transplant recipients

Abstract

Pneumocystis jirovecii pneumonia (PCP) incidence is increasing in kidney transplant recipients (KTR), but risk factors remain poorly defined. CD4+ T lymphopenia and mannose-binding lectin (MBL) deficiency are common immunodeficiencies in KTR. Here, we investigated whether CD4+ T lymphopenia and/or MBL deficiency would be risk factors for PCP in KTR. Furthermore, the role of thymic function in CD4+ T lymphopenia and outcome was studied by assessing CD45RA+CD31+CD4+ T cell numbers (RTE, recent thymus emigrants). In 321 de novo KTR serial determinations of peripheral T lymphocyte subsets (n=281, mean 4.2 times between days 0-365) and/or MBL levels (n=112, mean 1.8 times between days 30-180) were performed. 22/321 patients developed a PCP episode on average at day 199 (107-398) post-Tx. Age correlated inversely with RTE, CD4+ and CD8+ T-cell counts until day 180 post-Tx. RTE correlated with CD4+ T-cell counts at all time-points pre- and post-Tx. PCP patients had more CMV infections (p=0.045) within the first 3 months compared to controls. Importantly, PCP patients were older (p=0.008), and had lower RTE (p=0.046) pretransplant, and lower CD4+ T-cell counts pretransplant (p=0.017), at day 60 (p=0.032) and for the average of all post-Tx values (p=0.027) compared to controls. Though treatment with T-cell depleting antibodies was associated with consecutive CD4+ T lymphopenia in the whole cohort, the number of patients who received T-cell depleting antibodies was comparable between PCP and control patients (p=0.754). A multivariate stepwise logistic regression model identified only pretransplant CD4+ T-cell counts (OR 0.011, p=0.010) and acute rejection (OR 4.66, p=0.023) as predictors of PCP. In contrast, MBL levels and incidence of MBL deficiency (<500 ng/ml) at days 30, 90 and 180 post-Tx were not different between PCP patients and controls. In conclusion, PCP risk was associated with higher age and related to both thymic functional impairment and long-lasting CD4+ T-lymphopenia that started already before transplantation. Despite frequent occurrences in KTR, low levels of serum MBL were not associated with increased risk for PCP. CD4+ T-cell counts and function should be addressed in prospective studies for more individualized approaches to PCP prophylaxis.

Keywords: CD4+ T-lymphopenia; Kidney transplantation; Mannose binding lectin deficiency; Pneumocystis pneumonia.