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Monocytes are highly versatile innate immune cells responsible for pathogen clearance, innate immune coordination, and induction of adaptive immunity. Monocytes can directly and indirectly integrate pathogen-destructive instructions and contribute to disease control via pathogen uptake, presentation, or the release of cytokines. Indirect pathogen-specific instructions are conferred via Fc-receptor signaling and triggered by antibody opsonized material. Given the tremendous variation in polyclonal humoral immunity, defining the specific antibody-responses able to arm monocytes most effectively remains incompletely understood. While monocyte cell line-based assays have been used previously, cell lines may not faithfully recapitulate the full biology of monocytes. Thus, here we describe a multifaceted antigen-specific method for probing antibody-dependent primary monocyte phagocytosis (ADMP) and secondary responses. The assay not only reliably captures phagocytic uptake of immune complexes, but also detects unique changes in surface markers and cytokine secretions profiles, poorly detected by monocytic cell lines. The assay captures divergent polyclonal-monocyte recruiting activity across subjects with varying SARS-CoV-2 disease severity and also revealed biological nuances in Fc-mutant monoclonal antibody activity related to differences in Fc-receptor binding. Thus, the ADMP assay is a flexible assay able to provide key insights into the role of humoral immunity in driving monocyte phenotypic transitions and downstream functions across many diseases.

Respiratory syncytial virus (RSV) infection is a major cause of severe lower respiratory tract infection and death in young infants and the elderly. With no effective prophylactic treatment available, current vaccine candidates aim to elicit neutralizing antibodies. However, binding and neutralization have poorly predicted protection in the past, and accumulating data across epidemiologic cohorts and animal models collectively point to a role for additional antibody Fc-effector functions. To begin to define the humoral correlates of immunity against RSV, here we profiled an adenovirus 26 RSV-preF vaccine-induced humoral immune response in a group of healthy adults that were ultimately challenged with RSV. Protection from infection was linked to opsonophagocytic functions, driven by IgA and differentially glycosylated RSV-specific IgG profiles, marking a functional humoral immune signature of protection against RSV. Furthermore, Fc-modified monoclonal antibodies able to selectively recruit effector functions demonstrated significant antiviral control in a murine model of RSV.

Respiratory syncytial virus (RSV) infection is a major cause of severe respiratory illness in both young infants and the elderly. While antibodies represent key correlates of protection, the antibody mechanisms remain poorly understood. Emerging data point to additional humoral Fc-mediated mechanisms of action beyond neutralization. Therefore, to map the humoral correlates of immunity against RSV, antibody responses were profiled in a highly controlled non-human primate challenge model. Six different vaccine platforms were tested in African Green Monkeys, which post-challenge, yielded distinct antibody profiles and viral loads in both upper and lower respiratory tracts. Machine learning was then used to determine antibody features associated with protection. Upper respiratory control involved virus specific IgA levels, neutralization, and complement whereas lower respiratory control involved Fc-mediated mechanisms. These data highlight a collaborative Fab and Fc protective signature, induced selectively by distinct vaccine platforms, and provides mechanistic insights for the rational development of vaccines.

Summary As SARS-CoV-2 infections and death counts continue to rise, it remains unclear why some individuals recover from infection whereas others rapidly progress and die. While the immunological mechanisms that underlie different clinical trajectories remain poorly defined, pathogen-specific antibodies often point to immunological mechanisms of protection. Here, we profiled SARS-CoV-2–specific humoral responses on a cohort of 22 hospitalized individuals. Despite inter-individual heterogeneity, distinct antibody signatures resolved individuals with different outcomes. While no differences in SARS-CoV-2-specific IgG levels were observed, spike–specific humoral responses were enriched among convalescent individuals, whereas functional antibody responses to the nucleocapsid were elevated in deceased individuals. Furthermore, this enriched immunodominant S-specific antibody profile in convalescents was confirmed in a larger validation cohort. These results demonstrate that early antigen-specific and qualitative features of SARS-CoV-2-specific antibodies, point to differences in disease trajectory, highlighting the potential importance of functional antigen-specific humoral immunity to guide patient care and vaccine development., Graphical Abstract, Highlights • Limited early differences across groups were observed in titers and neutralization • Five antibody features collectively could differentiate convalescents and deceased • A shift in the balance of spike versus nucleocapsid immunity separated the groups • Spike-specific phagocytic and complement fixing activity was enriched in convalescents, Although most SARS-CoV-2 infected individuals experience mild disease, a significant fraction of individuals become severely infected. Early biomarkers that predict outcome are urgently needed. Atyeo et al demonstrate that distinct acute SARS-CoV-2 humoral immune responses exist across severely ill individuals that ultimately convalesce or pass away.

Understanding the properties and mechanisms by which antibodies provide protection is essential to defining immunity. Although neutralizing antibodies have been proposed as a potential key mechanism of protection against many viral pathogens, antibodies mediate additional immune functions that may have both protective and pathological consequences. Dissecting these properties against SARS-CoV-2 is likely necessary for defining metrics of immunity that will inform the design of vaccines and therapeutics and improve clinical management., Beyond neutralization, antibodies have immune-modulating functions that can be protective but, in some cases, can enhance pathology. Understanding these functions is critical for the development of safe vaccines and antibody therapies for COVID-19.

Respiratory syncytial virus (RSV) infection is a major cause of respiratory illness in infants and the elderly. Although several vaccines have been developed, none have succeeded in part due to our incomplete understanding of the correlates of immune protection. While both T cells and antibodies play a role, emerging data suggest that antibody-mediated mechanisms alone may be sufficient to provide protection. Therefore, to map the humoral correlates of immunity against RSV, antibody responses across six different vaccines were profiled in a highly controlled nonhuman primate-challenge model. Viral loads were monitored in both the upper and lower respiratory tracts, and machine learning was used to determine the vaccine platform-agnostic antibody features associated with protection. Upper respiratory control was associated with virus-specific IgA levels, neutralization, and complement activity, whereas lower respiratory control was associated with Fc-mediated effector mechanisms. These findings provide critical compartment-specific insights toward the rational development of future vaccines.

The urgent need for an effective SARS-CoV-2 vaccine has forced development to progress in the absence of well-defined correlates of immunity. While neutralization has been linked to protection against other pathogens, whether neutralization alone will be sufficient to drive protection against SARS-CoV-2 in the broader population remains unclear. Therefore, to fully define protective humoral immunity we dissected the early evolution of the humoral response in 193 hospitalized individuals ranging from moderate-to severe. Although robust IgM and IgA responses evolved in both survivors and non-survivors with severe disease, non-survivors showed attenuated IgG responses, accompanied by compromised Fcɣ-receptor binding and Fc-effector activity, pointing to deficient humoral development rather than disease-enhancing humoral immunity. In contrast, individuals with moderate disease exhibited delayed responses that ultimately matured. These data highlight distinct humoral trajectories associated with resolution of SARS-CoV-2 infection and the need for early functional humoral immunity., Highlights ● IgA and IgM evolve rapidly across all levels of disease severity ● Rapid and potent IgG class switching is linked to survival ● Moderate disease is associated with a delay but ultimate convergence of IgG ● Early S2-cross-reactivity is linked to survival after severe disease, Analyses of the functional humoral trajectories associated with the resolution of SARS-CoV-2 infection find that in spite of equivalent IgM and IgA immunity to the virus across all levels of disease severity, survival and recovery is linked to early class switching to IgG and the ability to leverage Fcγ-receptors targeting the spike protein.