By studying a total of 38 cases of PCP in 2110 KT recipients, we found that the majority of PCP cases occurred within 6 months after prophylaxis discontinuation, with 90.0% of patients who received rituximab experiencing PCP following discontinuation. The estimated NTT in the rituximab group was 29.0 in Rituximab group, whereas previous studies reported NTT values of 284 for aspirin in preventing cardiovascular events and 186 for statin in preventing myocardial infarction.[18,19] Collectively speaking, our study provides a strong evidence for prolongation of prophylaxis duration to 12 months in KT recipients treated with rituximab during perioperative period to prevent fatal infectious complications such as PCP.
In our study, the rituximab group included recipients treated with rituximab due to rejection episodes within 6 months after transplant as well as those who received rituximab for pre-operative desensitization. We included such patients because 6 months is considered a critical period of PCP occurrence ; accordingly, additional treatment with rituximab within 6 months after KT was found to be a significant risk factor for PCP in our cohort. Among the 14 patients who received rituximab for rejection treatment within 6 months following KT, PCP occurred in 9 patients (64.3 %) during few months after the discontinuation of TMP–SMX prophylaxis. Rejection within 6 months was significantly associated with PCP development in multivariate analysis, whether or not rituximab was used. Patients who underwent rejection treatment using rituximab had a 77.6-fold increased risk of PCP occurrence compared to the non-Rituximab group. Therefore, we strongly recommend that patients who undergo rejection treatment with rituximab should receive at least 6 months prophylaxis from the time of rejection treatment or 1year from transplant. Similar to the results of recent studies, CMV viremia prior to PCP was a significant risk factor in univariate analysis but not in multivariate analysis;[2,7,8] this result may be due to the higher rate of CMV viremia in the rituximab group. Thus, our results indicate that CMV viremia can be a good marker for patients’ immune status but not for the risk of PCP. The additional analysis for risk factors of PCP occurrence in the rituximab group compared to the non-rituximab group identified ABO incompatibility as a significant risk factor. Although it was not a study related to PCP, Sharif et al. showed that ABOi patients were more likely to experience BKVN compared to HLAi recipients.  Further investigation is needed to evaluate the possibility that intrinsic attributes of ABO incompatibility contribute to an increased risk for infectious complications after KT.
A total of 7 patients in the non-rituximab group developed PCP 1 year after transplant: 6 patients experienced PCP within 14 to 24 months after KT, and 1 patient developed PCP at 91 months after KT. Goto et al. suggested administering lifelong prophylaxis to prevent PCP occurrence; however, PCP prophylaxis for more than 1 year may not be appropriate considering the NNT value of 133.3 for prophylaxis prolongation from 6 to 12 months in our study and the low overall incidence of PCP after 1 year post-transplant. In the rituximab group, 6 patients developed PCP within 13 to 29 months after KT, all of whom had rejection treatment prior to PCP, including 4 patients who were treated with rituximab. Except for 2 patients who developed PCP at 7 months after discontinuation, 18 cases of PCP occurred within 6 months after prophylaxis discontinuation. Considering that PCP tended to occur within one year after KT in the rituximab group, we believe that with a proper duration of prophylaxis, PCP can be effectively prevented in patients who receive rituximab. Due to the limited number of patients, we could not analyze the effectiveness and necessity of 12-months prophylaxis after rituximab treatment for rejection treatment within 6 months after transplantation. Nevertheless, we suggest using 12-months prophylaxis for KT recipients who received rejection treatment, especially when they have other risk factors for PCP.
KT across the HLA and blood group A/B barriers has been recently increasing. Various desensitization protocols have been developed for such immunologically high-risk groups, and rituximab is one of the main component of pre-conditioning strategies and rejection treatment.[11,14,22] However, PCP in KT recipients treated with rituximab have not been evaluated extensively. In solid organ transplantation, it is hard to determine the impact of a single risk factor in PCP risk due to the confounding effects of various immunno-suppressive regimens and comorbidities of recipients. In our retrospective study using the database of a single large center, we showed that perioperative rituximab treatment had 3.09 times higher hazards of PCP occurrence after adjusting for other risk factors, which is in line with the results of previous studies.[2,5,7,8] Rituximab dose was suggested to be related with serious infectious complications following transplant,[11,12] with Lee et al. reporting that recipients treated with standard dose rituximab had higher risk of fungal infection than those who received lower dose of rituximab. In our results, rituximab dose did not show significant association with PCP occurrence; this result may be due to the abrupt and long-lasting effects of B-cell depression, and because the incidence of PCP was too small to obtain statistical significance.
CD4+ T lymphocytes orchestrate the defense against P. jiroveci, and low CD4+ T lymphocyte count is thus suggested as an independent risk factor associated with PCP in solid organ transplant recipients. In vivo studies have suggested a mechanism for how rituximab may increase the risk of PCP by inducing B-cell depletion:[23,24] Lind et al. showed that owing to the absence of P. jiroveci-specific antibody, mice with B-cell deficiency are more vulnerable to PCP, showing that as antigen presenting cells, B-cells play an important role in the defense response against P. jiroveci. The same group also reported that B- and T-cell interaction carries a vital role in generating effector and memory CD4+ T lymphocyte response against P. jiroveci. In addition, clinical studies on patients with hematologic malignancies supported the theory that B-cell suppression using rituximab increases the risk of PCP development.[13,25]
Recent studies showed that rituximab results in long-term elimination of B-cells up to more than 6 months, thereby suggesting prolongation of prophylaxis.[9,10] Sidnet et al. reported that a single dose of rituximab in sensitized patients awaiting KT can induce rapid depletion of B-cell, which was maintained from 6 months to 1 year. In addition, repopulation of functional B-cell subsets against microorganisms was predominantly preceded by CD19+CD5+ polyreactive B-cells and ontogenetically younger B-cells with reacting low affinity antibodies. Ganberg et al. studied the effect of rituximab on B-cell populations in peripheral blood, within kidney biopsy tissues, and in inguinal lymph nodes in KT recipients who were maintained in conventional triple immunosuppressants; the authors showed that although the maximal effect was observed between 3 weeks to 6 months, B-cell populations remained suppressed up to several years. In ABOi KT recipients, CD19+ cells did not recover after 12 months even after a single injection of reduced dose rituximab (200 mg). Our results further support the results of these studies and advocate the use of prolonged prophylaxis for 12 months.
This study is limited in that it was a retrospective study performed at a single center, which may have resulted in selection and information biases. Nevertheless, such study design also resulted in homogeneity of both study population and immunosuppressive protocol. Also, as most of the patients were of Asian descent, our results may have limited generalizability in other races. Lastly, basiliximab was primarily used as an induction treatment rather than ATG, especially in the rituximab group; although ATG was not a significant risk factor for PCP in our study, the incidence of PCP may be different in other clinical settings with different induction treatment protocols.