COVID-19: B-Cell Depletion and Sepsis Related Changes in Bone Marrow and Spleen

Introduction: By now, the pandemic spread of COVID-19 (coronavirus disease 2019) has claimed more than 600,000 lives. The adaptive immune response seems to play a major role in the progression of the disease, since entry of SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2) is determined by a spike protein recognized by T helper cells. This has been linked to the clinical finding of severe lymphocytopenia in these patients. However, detailed cellular immune responses in the bone marrow (BM) and in the spleen (SPL) of COVID-19 patients have not been addressed yet. Here, we provide novel immunologic insight with potential for therapeutic management and risk stratification in COVID-19.

Material and Methods: We performed complete autopsies on 11 confirmed COVID-19 and 4 non-COVID-19 deceased, who were matched for risk profile and age. SARS-CoV-2 load was measured by rt-PCR (quantitative real-time polymerase chain reaction) targeting the SARS-CoV-2 E-gene in purified RNA extracts from 50mg of pulmonary tissue (MagNAPure 96 system, Viral NA Large Volume Kit, Roche). For histopathology, representative tissue samples of decalcified BM and SPL were fixed in 4 % buffered formalin, dehydrated and paraffin embedded. Sections were stained with HE, PAS, Giemsa-, Gomori- and Prussian blue stain. Furthermore, BM and SPL were stained with immunohistochemical reagents, namely MPO (Myeloperoxidase), CD235, CD34, CD117, CD68, CD61, CD20, CD3, CD4, CD8, CD138, HLA-DR (Human Leucocyte antigen - DR isotype), PD-1, PD-L1 (Programmed cell death protein and ligand 1), Ki67 and Caspase3 (Ventana Ultra and LEICA Bond III). Additionally, we performed in-situ hybridization of EBV (Epstein-Barr-Virus; LEICA Bond MAX), followed by PCR of the EBV nuclear antigen 1 (Thermo Fisher and Roche). Histopathology was evaluated by at least two hematopathologists. Clinical data were obtained from patients’ files. Statistical analysis was done using GraphPad Prism8 Software. Inc, 2018.

Results: Of all COVID-19 deceased, 73% (n = 8/11) showed BM hypercellularity, increased granulocyte / erythrocyte ratios, and left shift of erythro- and granulopoiesis with anemia and an increase of immature granulocytes in the peripheral blood. Thromboembolic events were present in 82% (n = 9/11) of COVID-19 patients and related to an increase and left shift of megakaryopoiesis in the BM. In the BM of patients with severe bacterial superinfection of COVID-19 pneumonia, we observed an early increase of PD-L1 expression on myeloid cells, lymphocytic apoptosis, and time-dependent macrophage anergy with a continuous loss of antigen-presenting capacity. Furthermore, we found CD20+ B-cell depletion in either BM or SPL in 64% (n = 7/11) of COVID-19 patients with B-cell counts of less than 1% in the BM and 1-5% in the SPL, followed by complete plasma cell depletion. This was reflected by severe lymphocytopenia in the peripheral blood. In contrast, BM T-cell counts were nearly as high in COVID-19 decedents (median 10%) as in cases not related to COVID-19 (median 12.5%). Interestingly, there was a tendency towards higher pulmonary SARS-CoV-2 RNA load in COVID-19 patients with B-cell depletion, as we observed maximum viral copy numbers of up to 1,150,000 / 10,000 cells in patients with B-cell depletion as compared to 6,500 / 10,000 cells in patients with B-cell preservation. EBV was absent in all cases. Clinical characteristics and time-intervals between initial symptoms and death of COVID-19 patients were heterogenous, therefore preventing the detection of a clinical risk profile in patients with B-cell depletion.

Conclusion: Our results show that severe lymphocyte depletion in COVID-19 deceased is caused by a substantial loss of B-cells which is in turn associated with viral SARS-CoV-2 burden and presumably results from excessive activation of the adaptive immune system. It is yet to be determined how B-cell specific pathways are affected by SARS-CoV-2 and whether this might serve as a therapeutic target of interest. Moreover, we provide morphologic evidence, that COVID-19 pneumonia with bacterial superinfection is aggravated by sepsis acquired immunodeficiency. Since the latter is associated with an epigenetically determined switch to endotoxin tolerance, our findings may additionally aid in risk stratification of COVID-19 patients who undergo severe bacterial superinfection during the disease.