sMAdCAM:IL-6 (sMIL Index): A novel signature associated with COVID-19 disease progression and development of anti-SARS-CoV-2 antibodies

Jagtap, D.; Bhor, V. M.; Bhowmick, S.; Kasarpalkar, N.; Sagvekar, P.; Kulkarni, B.; Pathak, M.; Chatterjee, N.; Dolas, P.; Palav, H.; Kaginkar, S.; Bhagat, S.; Munshi, I.; Parikh, S.; Agrawal, S.; Pawar, C.; Kaneria, M.; Mahale, S.; Shastri, J.; Patel, V.

IMPORTANCE: Recent studies positing the gut as a sanctuary site for viral persistence in SARS-CoV-2 infection highlight the importance of assimilating profiles of systemic as well as gut inflammatory mediators to understand the pathology of COVID-19. Also, the role of these markers in governing virus specific immunity following infection remains largely unexplored. OBJECTIVE: To evaluate the role of systemic and gut inflammatory markers in disease progression and development of anti-viral humoral immunity following SARS-CoV-2 infection. DESIGN, SETTING AND PARTICIPANTS: This cohort study (n=58) of SARS-CoV-2 infected individuals included a group of in-patients (n=36) at various stages of disease progression together with convalescent individuals (n=22) recruited between April and June 2020 (peak of the epidemic) from a tertiary care hospital in Mumbai, India. Follow-up of 11 in-patients at day 7 post diagnosis was carried out, resulting in a total of 47 in-patient samples. EXPOSURES: Diagnosis of SARS-CoV-2 infections was confirmed by reverse transcriptase-polymerase chain reaction-based testing of nasopharyngeal/oropharyngeal samples. MAIN OUTCOMES AND MEASURES: Primary outcomes were the measurement of inflammatory markers including Th1/Th2/Th17 cytokines and levels of soluble mucosal addressin cell adhesion molecule (sMAdCAM) in plasma. Anti-viral humoral response was measured by rapid antibody test (IgG, IgM) and chemiluminescent immunoassay (CLIA) (IgG). Also antibodies binding to SARS-CoV-2 proteins were measured by surface plasmon resonance (SPR). Secondary outcomes were correlation of the inflammatory signature with clinical information, including age, sex, disease duration and co-morbidities. RESULTS: Twenty eight of 36 (78%) in-patients and 19 of 22 (86%) convalescents were males. Out of 47 in-patient samples, 22 (46%), 11 (23%) and 14 (30%) were IgG-/IgM-, IgG+/IgM+ and IgG+/IgM- respectively. Of 22 convalescent samples, 3 (14%), 1 (4%) and 17 (77%) were respectively IgG-/IgM-, IgG+/IgM+ and IgG+/IgM-. Two out of 22 (9%) convalescents showed high IL-6 levels (>100pg/ml) and 4 (18%) had high TNF levels (>30pg/ml). However, the convalescents (n =22) had significantly lower levels of IL-6 [Median=27.48 (IQR=23.54-39.92)] compared to followed up in-patients (n = 11) at day 0 [Median=111(IQR=68-129.7), p =0.0002] and higher levels of sMAdCAM [Median=1940 (1711-2174) pg/ml] compared to these individuals at day 0 [Median=1701 (IQR=1532-1836) pg/ml; p=0.032] and day 7 [Median=1534 (IQR=1236-1654) pg/ml; p=0.0007]. Further, IL-6 and sMAdCAM levels among in-patients inversely correlated with one another (r =-0.374, p = 0.009, CI = 95%). When expressed as a novel integrated marker, sMIL (sMAdCAM/IL-6 ratio) index, these levels were incrementally and significantly higher across various disease states with convalescents exhibiting the highest values [Median= 64.74 (IQR=47.33-85.58)]. Also, the sMIL index was significantly higher in convalescents (with class-switched responses) compared to IgG+/IgM+ individuals at early stages of infection [Median=28.65 (IQR=13.63-96.26), p = 0.034]. Real-time measurement by SPR of plasma antibody binding to viral nucleocapsid (NC), receptor binding domain (RBD) and spike (S) revealed waxing and waning of plasma antibody responses to all 3 targets. Importantly, sMAdCAM levels as well as sMIL index (fold change) correlated with peak association rates of RBD-binding (r = 0.462, p = 0.03, CI = 95%) and fold change in binding to S (r = 0.68, p = 0.050, CI = 95%) respectively. CONCLUSION AND RELEVANCE: Our results highlight key systemic and gut-associated immune parameters that need to be monitored and investigated further to optimally guide therapeutic and prophylactic interventions for COVID-19.