An Exploratory Clinicopathological Study Investigating Immunohistochemical Markers (CD61 and Fibrin) for Detecting Platelet Aggregates and Fibrin Deposition in Lupus Nephritis.

OBJECTIVES: To evaluate platelet aggregates and fibrin deposition in kidney biopsies of patients with lupus nephritis (LN) and to identify associated clinicopathological factors. METHODS: Initial kidney biopsies of LN, classified according to the 2003 ISN/RPS criteria, were retrospectively analyzed. Specimens were immunostained with CD61 for platelet aggregates and fibrin antibodies for fibrin deposition. Associations with clinical, laboratory, and histopathological parameters were examined. RESULTS: A total of 44 patients with biopsy-proven LN were included, of whom 37 (84%) were female. The mean age was 41.5 ± 14.1 years, and the median follow-up duration was 32 months (IQR, 9-73.5). CD61 and fibrin positivity were observed in 56.8% and 45.5% of cases, respectively, predominantly in glomeruli. CD61 positivity was associated with low complement 4 levels at kidney biopsy (OR: 4.50, 95% CI: 1.10-18.27; p = 0.035). The mean cellular crescent score was higher in the CD61-positive group than in the CD61-negative group (0.33 ± 0.56 vs. 0.05 ± 0.22, p = 0.047). Additionally, the fibrin-positive group showed a higher mean chronicity index compared to the fibrin-negative group (2.90 ± 1.16 vs. 2.30 ± 1.25). Fibrin positivity was associated with alopecia, antiphospholipid antibody positivity (including lupus anticoagulant or anticardiolipin IgG and/or IgM) in univariable analysis. CONCLUSIONS: CD61-positive platelet aggregates and fibrin deposits were commonly seen in LN biopsies and were linked to specific clinical and pathological features. Nevertheless, these should not be considered equivalent to true microthrombi and might indicate a combination of inflammatory and thrombotic processes causing microvascular injury. Further validation is warranted.