B43-35 Low CD4 and a Red Herring: A Case of Mixed Connective Tissue Disease Masked by Pneumocystis Jirovecii Pneumonia

Abstract Introduction Mixed Connective Tissue Disease (MCTD), commonly seen in middle-aged females, is frequently associated with ILD and leukopenia. It can also predispose to opportunistic infections such as Pneumocystis jirovecii Pneumonia (PJP), which often presents with overlapping pulmonary features. Case Presentation A 62-year-old Chinese male (nonsmoker) presented to the pulmonary clinic for chronic intermittent cough with small-volume hemoptysis without systemic symptoms. Chest CT revealed paraseptal emphysema, diffuse cysts predominantly in the right upper lobe, patchy ground-glass opacities (GGOs), and interlobular septal thickening bilaterally. PFT was normal, and BAL was positive for PJP by PCR, with old hemorrhage and mild scarring on biopsy. A low CD4 count of 399 cells/μL (24%) was detected, which was presumed idiopathic (ICL) after ruling out primary and secondary immunodeficiency. Subsequently, empiric short-course steroids were prescribed, and PJP was treated with Trimethoprim-sulfamethoxazole (TMP-SMX) for 21 days, followed by a 3-month prophylaxis. 2 weeks after its completion, he developed acute hypoxemic respiratory failure that progressed despite broad-spectrum antibiotics. He was intubated and empirically started on TMP-SMX and IV steroids for presumed severe PJP. Chest CT revealed relative worsening of GGOs. An extensive infectious and autoimmune workup done during the ICU stay was negative, including PJP on BAL analysis. Since oxygenation improved with high-dose steroids, he was discharged with a 6-week taper. 2 months later, a repeat autoimmune panel for recurrent hemoptysis was positive for ANA/anti-U1 RNP antibodies. Mixed Connective Tissue Disease (MCTD) was diagnosed, and prednisone and mycophenolate were started with excellent response. Discussion Around 56% MCTD cases have ILD, most commonly a non-specific interstitial pattern. While our case’s initial CT had features of ILD, the thin-walled diffuse cysts with patchy GGOs, especially with a positive PCR, were highly suggestive of PJP. Furthermore, lack of systemic symptoms, initial ANA seronegativity, and rarity of MCTD (incidence 0.2-1.9/100,000), especially in older men, made early diagnosis challenging. Additionally, while leukopenia occurs in ∼75% MCTD cases, isolated CD4 lymphocytopenia (ICL) has not been reported, except for one study that found an imbalance in CD4 subsets (Low proportion of CD4+CD9+ cells). ICL justified recurrent PJP; however, autoimmune rheumatic disorders can also significantly increase the overall risk of PJP, as seen here. This diagnostically challenging case of an older male with hemoptysis initially presenting with recurrent PJP in the setting of presumed ICL and later revealing MCTD emphasizes the importance of broader differential, close follow-up, and repeat testing for autoimmune disorders in the right clinical setting. This abstract is funded by: None