Hidden consequences: Uncovering belimumab's role in the development of Pneumocystis jirovecii pneumonia in a lupus patient.

Our report demonstrates the first confirmed case of pneumocystis jirovecii pneumonia in a patient with B cell depletion due to chronic maintenance therapy with belimumab. Our patient's diagnosis of PJP was unexpected, given her normal CD4+ count and the use of such a low dose of prednisone. We attribute her susceptibility to PJP infection to her profound B cell depletion in response to her belimumab therapy, supporting previous research indicating the importance of CD4+ T cells and B cells in the protective immune response against PJP. This case may help shape future clinical guidelines concerning PJP prophylaxis, particularly in SLE patients with deficient B lymphocytic activity and B cell depletion.