Differences in clinical Pneumocystis pneumonia in rheumatoid arthritis and other connective tissue diseases suggesting a rheumatoid-specific interstitial lung injury spectrum

To compare Pneumocystis pneumonia (PCP) in patients with rheumatoid arthritis (RA) with PCP in patients with non-RA connective tissue diseases (CTDs) in order to clarify the characteristics of the former. We extracted consecutive patients satisfying the following criteria for “clinical PCP”: (1) positive plasma β-d-glucan, (2) PCP-compatible computed tomography findings of the lung, and (3) successful treatment with antipneumocystic antibiotics. Patients who underwent methylprednisolone “pulse” therapy or sufficient antibiotics to cure bacterial pneumonia were excluded. We used the t test, U test, or Fischer’s exact probability test to compare the two groups and Jonckheere-Terpstra’s test and Ryan’s procedure for the trend test. Thirty-five cases were extracted. The underlying rheumatic diseases were RA in 25 and non-RA CTDs in ten. At the onset of clinical PCP, the lymphocyte counts were 884 vs 357/mm³ (p < 0.001), PC-PCR positivity 64% vs 100% (p = 0.029), glucocorticoid dose 4.0 vs 17.5 mg PSL/day (p < 0.001), and methotrexate dose 8 vs 0 mg/week (p = 0.003). The PC-PCR-negative patients, observed only in the RA group, were all receiving methotrexate (MTX) therapy except one patient who was receiving high-dose prednisolone alone. All PC-PCR-positive patients were receiving glucocorticoid, TNF inhibitor, or a non-MTX immunosuppressant. No patient with MTX alone had positive PC-PCR results. Clinical PCP in RA patients differed from that in non-RA CTD patients and may be understood as only a part of the rheumatoid-specific interstitial lung injury spectrum influenced by multiple, synergistic factors including MTX, Pneumocystis, and RA itself.