Resolution of viral load in mild COVID-19 patients is associated with both innate and adaptive immune responses

Abstract

Over 90% of the COVID-19 patients manifest mild/moderate symptoms or are asymptomatic. Although comorbidities and dysregulation of immune response have been implicated in severe COVID-19, the host factors that associate with asymptomatic or mild infections have not been characterized. We have collected serial samples from 23 hospitalized COVID-19 patients with mild symptoms and measured the kinetics of SARS-CoV-2 viral load in respiratory samples and markers of inflammation in serum samples. We monitored seroconversion during the acute phase of illness and quantitated the amount of total IgG against the receptor-binding domain of SARS-CoV-2 and estimated the virus neutralization potential of these antibodies. Viral load decreased by day 8 in all the patients but the detection of viral RNA in saliva samples did not correlate well with viral RNA detection in nasopharyngeal/oropharyngeal swab samples. 25% of the virus-positive patients had no detectable neutralizing antibodies in the serum and in other cases, the efficiency of antibodies to neutralize SARS-CoV-2 B1.1.7 strain was lower as compared to the circulating virus isolate. Decrease in viral load coincided with increase in neutralizing antibodies and interferon levels in serum. Most patients showed no increase in inflammatory cytokines such as IL-1β or IL-6, however, elevated levels of IL-7 and other inflammatory mediators such as TNF-α and IL-8 was observed. These data suggest that most mild infections are associated with absence of inflammation coupled with an active innate immune response, T-cell activation and neutralizing antibodies.

Keywords: Antibodies; COVID-19; Cytokines; Inflammation; PRNT; SARS-CoV-2.

Fig. 1

SARS-CoV-2 RNA detection in serial samples of hospitalized patients.(A-P) Total RNA was isolated from virus transport medium containing nasopharyngeal/oropharyngeal swabs or from saliva samples collected on indicated days from hospitalized COVID-19 patients. SARS-CoV-2 N gene was detected by quantitative RT-PCR. Respective patient numbers are indicated.

Fig. 2

Quantitation of RBD antibodies and PRNT50 titers. (A-P) Serum samples collected on the day-of-admission (DOA) and day-of-discharge (DOD) from the hospital were used to estimate antibodies against the RBD of SARS-CoV-2 spike protein by quantitaive ELISA. The same sample was also used to determine the PRNT50 titers using wild type virus neutralization assay. Data was expressed as international units as the reference reagent was calibrated against the WHO international reference reagent.

Fig. 3

Quantitation of serum interferons and inflammatory markers. (A-F) Levels of indicated analytes in serum samples collected serially from hospitalizedpatients was estimated by multiplex luminex bead assays. Bar represents the median values. Healthy and other febrile illness samples were from a single time-point.

Fig. 4

Quantitation of serum chemokines. (A-D) Levels of indicated analytes in serum samples collected serially from hospitalized patients was estimated by multiplex luminex bead assays. Bar represents the median values. Healthy and other febrile illness samples were from a single time-point. P values were determined by ANOVA.