Unanticipated Global Emergence of Multiple Pneumocystis jirovecii Mutants Selected by Mycophenolic Acid Driving Increasing Outbreaks in Solid Organ Transplant Recipients.

Background: Classified by the WHO as one of the 19 most dangerous fungal pathogens, Pneumocystis jirovecii has been associated with increasing outbreaks of Pneumocystis pneumonia (PCP) among solid organ transplant (SOT) recipients worldwide. Mycophenolic acid (MPA), an inosine monophosphate dehydrogenase (IMPDH) inhibitor commonly used as an immunosuppressant to prevent organ rejection, is a risk factor for PCP. However, MPA also displays antifungal activity, potentially protecting against PCP, despite not being used to treat it. Therefore the underlying factors driving these outbreaks remain undefined.

Methods: In this international multicenter retrospective observational study, P. jirovecii samples were collected from 96 SOT patients (including 94 from nine separate outbreaks and 84 on MPA therapy) and 67 non-transplant controls (none on MPA), between 1986 and 2020 across six countries in Europe, North America and Asia. All samples underwent extensive targeted sequencing of the P. jirovecii inosine monophosphate dehydrogenase (impdh) gene and multiple genetic markers, with selected samples further analyzed for complete mitogenome and restriction fragment length polymorphisms. Computational modeling was employed to predict the effects of IMPDH mutations on protein structure and MPA binding.

Results: Six impdh mutations (including one previously reported) were identified, with frequencies of 4-21% each in SOT patients and 0-1% in controls. These mutations were strongly associated with prior MPA exposure and showed marked geographic segregation and temporal shifts. Four mutations were each linked to multiple distinct genotype profiles, representing separate P. jirovecii strains. Structure modeling predicted that these four mutations reduced protein stability and binding affinity to MPA.

Conclusions: This study suggests that the widespread use of MPA in SOT recipients has unexpectedly driven the emergence of multiple impdh mutations in P. jirovecii, each presumably arising independently in multiple strains worldwide. These mutations likely confer drug resistance and provide a selective survival advantage to P. jirovecii in SOT recipients exposed to MPA, thereby facilitating transmission and outbreaks. These findings have significant implications for the prevention and clinical management of PCP in SOT recipients, highlighting a rare example of how antimicrobial resistance can emerge through unexpected pathways, transcending conventional antimicrobial use and emphasizing the need for increased vigilance and strategic adaptation in clinical practice.