Characterization of Complement and Immunoglobulin Deposition in Bone Marrow Trephine Biopsies of Patients with Cytopenias

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Cytopenias can be categorized in “peripheral”, such as autoimmune hemolytic anemias (AIHA), immune thrombocytopenic purpura (ITP), and chronic idiopathic/autoimmune neutropenia (CIN/AIN), and “central”, including disorders such as aplastic anemia (AA) and myelodysplastic neoplasms (MDS). Several overlaps exist between the two forms, with a clear role of the humoral and complement immune response in the former, that is however less known in the latter. We assessed complement fractions C3 and C4, IgM and IgG deposits by immunohistochemistry on bone marrow biopsies from 162 cytopenic patients diagnosed between 2016 and 2023 at a tertiary hematologic center, reference for immune cytopenias, in Milan, Italy. Deposits were expressed using a semiquantitative scale from 0 to +3 by an expert hemopathologist. The topographical distribution was then evaluated and categorized as serous versus precursors pattern. We collected laboratory data at diagnosis to evaluate a correlation with the immunohistochemical data. Overall, 28 AIHA, 15 ITP, 58 CIN/AIN, 12 AA, and 49 lower-risk MDS were accrued. A significantly greater deposition of C3 and C4 (≥2+) was found in AA and MDS versus peripheral cytopenias (42% and 33% in AA, 49% and 41% in MDS versus 21% and 14% in all peripheral cytopenias, p=0.004 and p=0.002, respectively). Ig deposition was significantly greater (≥ 3+) in MDS and AA (63% and 42% for IgM and 49% and 33% for IgG, respectively) versus peripheral cytopenias (39% for IgM and 11% for IgG, respectively, p < 0.05); notably, IgM deposition was particularly evident in AIHA, ≥ 3+ in 50% of cases. Topographically, most of C3 and C4 deposition showed a serous pattern, while deposition on precursors was observed in 25% of MDS cases (mainly C3) and 42% of AIHA (mainly C4). IgG/M deposits were equally distributed on serum and precursors in all disease category. Hb and reticulocytes values were negatively correlated with C4 deposits in MDS patients (r= -0.031, p=0.01 for Hb and r= -0.29, p=0.02 for reticulocytes). Bone marrow cellularity negatively correlated with IgM deposits in AA (r= -0.72, p=0.004). Finally, platelet counts negatively correlated with IgM deposits in ITP (r=-0.5, p=0.02). In conclusion, our data demonstrated a significant deposition of complement and Ig in bone marrow of patients with “peripheral” and “central” cytopenias, more marked in the latter. The correlation with hematological parameters hints for a role of the immune attack against bone marrow precursors in all these conditions and might foster innovative therapeutic approaches.

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