Serum Fc-Mediated Monocyte Phagocytosis Activity Is Stable for Several Months after SARS-CoV-2 Asymptomatic and Mildly Symptomatic Infection

We investigated the temporal profile of multiple components of the serological response after asymptomatic or mildly symptomatic SARS-CoV-2 infection, in a cohort of 67 previously SARS-CoV-2 naive young adults, up to 8.5 months after infection. We found a significant decrease of spike IgG and neutralization antibody titers from early (11 to 56 days) to late (4 to 8.5 months) time points postinfection. Over the study period, S1-specific IgG levels declined significantly faster than that of the S2-specific IgG. Further, serum antibodies from PCR-confirmed participants cross-recognized S2, but not S1, of the betacoronaviruses HKU1 and OC43, suggesting a greater degree of cross-reactivity of S2 among betacoronaviruses. Antibody-Dependent Natural Killer cell Activation (ADNKA) was detected at the early time point but significantly decreased at the late time point. Induction of serum Antibody-Dependent Monocyte Phagocytosis (ADMP) was detected in all the infected participants, and its levels remained stable over time. Additionally, a reduced percentage of participants had detectable neutralizing activity against the Beta (50%), Gamma (61 to 67%), and Delta (90 to 94%) variants, both early and late postinfection, compared to the ancestral strain (100%). Antibody binding to S1 and RBD of Beta, Gamma, Delta (1.7 to 2.3-fold decrease), and Omicron (10 to 16-fold decrease) variants was also significantly reduced compared to the ancestral SARS-CoV-2 strain. Overall, we found variable temporal profiles of specific components and functionality of the serological response to SARS-CoV-2 in young adults, which is characterized by lasting, but decreased, neutralizing activity and antibody binding to S1, stable ADMP activity, and relatively stable S2-specific IgG levels. IMPORTANCE Adaptive immunity mediated by antibodies is important for controlling SARS-CoV-2 infection. While vaccines against COVID-19 are currently widely distributed, a high proportion of the global population is still unvaccinated. Therefore, understanding the dynamics and maintenance of the naive humoral immune response to SARS-CoV-2 is of great importance. In addition, long-term responses after asymptomatic infection are not well-characterized, given the challenges in identifying such cases. Here, we investigated the longitudinal humoral profile in a well-characterized cohort of young adults with documented asymptomatic or mildly symptomatic SARS-CoV-2 infection. By analyzing samples collected preinfection, early after infection and during late convalescence, we found that, while neutralizing activity decreased over time, high levels of serum S2 IgG and Antibody-Dependent Monocyte Phagocytosis (ADMP) activity were maintained up to 8.5 months after infection. This suggests that a subset of antibodies with specific functions could contribute to long-term protection against SARS-CoV-2 in convalescent unvaccinated individuals.