Children and youth infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) have milder disease than do adults, and even among those with the recently described multisystem inflammatory syndrome, mortality is rare. The reasons for the differences in clinical manifestations are unknown but suggest that age-dependent factors may modulate the antiviral immune response. We compared cytokine, humoral, and cellular immune responses in pediatric (children and youth, age <24 years) (n = 65) and adult (n = 60) patients with coronavirus disease 2019 (COVID-19) at a metropolitan hospital system in New York City. The pediatric patients had a shorter length of stay, decreased requirement for mechanical ventilation, and lower mortality compared to adults. The serum concentrations of interleukin-17A (IL-17A) and interferon-γ (IFN-γ), but not tumor necrosis factor–α (TNF-α) or IL-6, were inversely related to age. Adults mounted a more robust T cell response to the viral spike protein compared to pediatric patients as evidenced by increased expression of CD25+ on CD4+ T cells and the frequency of IFN-γ+ CD4+ T cells. Moreover, serum neutralizing antibody titers and antibody-dependent cellular phagocytosis were higher in adults compared to pediatric patients with COVID-19. The neutralizing antibody titer correlated positively with age and negatively with IL-17A and IFN-γ serum concentrations. There were no differences in anti-spike protein antibody titers to other human coronaviruses. Together, these findings demonstrate that the poor outcome in hospitalized adults with COVID-19 compared to children may not be attributable to a failure to generate adaptive immune responses.
The clinical manifestations and outcomes from COVID-19 differ by age as evidenced by a shorter length of hospital stay, decreased requirement for mechanical ventilation, and decreased mortality in children (including those who presented with MIS-C) compared to adults. Results of our study indicate that the poorer outcomes in adults were not attributable to a failure to generate T cell or antibody responses to the virus. The opposite was observed as adults had higher neutralizing antibody titers, ADCP, and more vigorous T cell responses to viral spike proteins compared to pediatric patients. Immunity to other common cold human CoVs has been speculated to provide cross-protection or to promote antibody-mediated enhancement. However, we did not identify differences in preexisting antibodies to other human CoVs by age or outcome, suggesting that they do not play a dominant role in modulating the clinical response.
Further dissection of the immune responses identified several differences that may provide insights into how host responses contribute to clinical outcomes. Specifically, pediatric patients had higher serum concentrations of IL-17A and IFN-γ shortly after presentation. This age-associated difference was most notable for IL-17A, which persisted even after excluding the patients with MIS-C who were delayed in their hospitalization from the initial SARS-CoV-2 exposure compared to patients in the other groups. This observation suggests that IL-17A or the cells that produce it may contribute to immune protection, particularly against lung disease, which was milder in group 1 and an uncommon manifestation of MIS-C. IL-17A is produced by multiple cell types including CD4+ T cells, CD8+ T cells, γd T cells, invariant natural killer T (NKT) cells, innate lymphoid cells, and neutrophils. We did not find a robust IL-17A response, measured by intracellular cytokine staining in CD4+ T cells, at times when we could detect intracellular IFN-γ, suggesting that other cells may be the primary source of IL-17A in serum. Likewise, the frequencies of IFN-γ+ CD4+ T cells were highest in samples obtained from the adult patients (groups 3 and 4), suggesting that other cells may be the source of this cytokine in the serum. Our findings are consistent with a recent report in which IFN-γ, but not IL-17A, was detected in T cells from patients with COVID-19 that were stimulated with peptide from the virus.
Several preclinical and clinical studies of bronchoalveolar lavage (BAL) samples from patients with other respiratory infections and diseases have identified tissue-resident immune cells that may produce IL-17A and IFN-γ, although the specific role of these cytokines is not clear. Dual cytokine (IL-17A and IFN-γ)–producing resident memory cells have been described in the lung, and a protective role for T helper 17 cells has been described in pulmonary infections. In addition, pulmonary epithelial cells have been shown to be a source of IFN-γ during infection with Mycobacterium tuberculosis. Possibly, the higher concentrations of IFN-γ and IL-17A in the serum of pediatric patients with COVID-19 reflected increased expression by cells in the respiratory tract, and these local cytokines may have protected the patients from progressive respiratory disease. The cytokines may have contributed to more rapid clearance of the virus, although we did not obtain quantitative SARS-CoV-2 polymerase chain reaction (PCR) data, longitudinal nasopharyngeal swabs, or BAL samples to test this directly. Virus clearance is suggested by the observation that half of the patients with MIS-C did not have viral RNA detected in their nasal swabs at the time of presentation.
The age-related difference in IL-17A concentrations in serum is consistent with described dysfunction of innate immune responses in older individuals. Decreased expression of pattern recognition receptors such as retinoic-acid inducible gene-1 (RIG-I) by monocytes has been found with aging and has been postulated to account for reduced type 1 IFN release. Invariant NKT cells, which secrete IL-17A and have cytolytic function, also decrease in number and function with aging.
A consequence of a more robust innate immune response in children might be a diminished adaptive immune response. This notion is supported by the observation of a lower frequency of antigen-reactive (spike protein) CD25+ and IFN-γ–producing CD4+ T cells after stimulation with spike protein, lower neutralizing antibody titers, and less ADCP activity in pediatric compared to adult patients, and an inverse correlation between neutralizing antibody titer and serum concentrations of IL-17A and IFN-γ. Group 1 pediatric patients with COVID-19 who recovered without sequelae exhibited the lowest ADCP activity and had the lowest serum concentrations of IL-6 and TNF, cytokines associated with ARDS, and poor outcome in clinical studies. These observations raise the possibility that release of cytokines by Fc receptor–bearing cells during phagocytosis may contribute to the inflammatory cytokine storm linked to progressive ARDS in adults or MIS-C in children. The notion that ADCP may contribute to enhanced pathology has been suggested in previous studies with other viruses, most notably dengue virus and the related flavivirus, Zika virus. The findings are also reminiscent of earlier studies in adults with SARS-CoV-1 infection, where both neutralizing antibody and ADCP responses were higher in patients who died compared to those who recovered. As with the current studies, Fc receptor blockade inhibited the response, indicating that the phagocytic activity was mediated by interactions between the antibodies and Fc receptors.
Whereas neither the quantity nor functionality of antibody responses differed when comparing the adults by outcome, differences were observed between the pediatric cohorts. Specifically, the pediatric patients with MIS-C (group 2) had a greater proportion of IgG1 versus IgG3 spike-specific antibodies and more ADCP activity compared to non–MIS-C pediatric patients. Differences in antibody function may be due to subclass, Fc glycans, and antigenic targets. The differences in the relative proportion of IgG1 and IgG3 in patients with MIS-C may also contribute to the differences in immune responses directed at other viral proteins including membrane or nucleocapsid proteins, which were not evaluated in the current study.
The results from our study provide new insights into potential mechanisms that may contribute to age-related differences in disease resolution or enhanced pathology in patients with COVID-19 and may have implications for ongoing efforts with convalescent plasma and the development of therapeutic antibodies. The finding that adults who did poorly had high titers of antibodies that were both neutralizing and induced phagocytosis, as well as the greater T cell responses suggest that boosting of these functional responses to the spike protein, particularly late in the course of disease, may not be beneficial. In contrast, boosting of early innate immune responses may be important.
There are a number of limitations to our study. The patients and their immunologic measurements were heterogeneous, and therefore, differences could be missed because of the variance within the groups and the small sample size. Some of the patients received brief treatment with hydroxychloroquine, remdesivir, methylprednisolone, IVIG, or other therapies during the course of their hospitalization, and we cannot exclude an effect of these agents on cellular immune responses. In addition, we did not have access to BAL or tissue from the patients, and there may be findings in the lung or regional lymph nodes that are not reflected in the peripheral blood. Last, we did not have access to serial samples to study the kinetics of the immune responses or the release of other mediators such as type 1 IFNs. We cannot track the exact time of exposure to SARS-CoV-2, and we did not have results for viral shedding so there may have been differences in immune responses during the clinical course that went undetected. Our assessment of progression through the disease course could only be assessed clinically.
In summary, we have identified differences in humoral and cellular immune responses to SARS-CoV-2 infection between pediatric and adult patients with COVID-19 and have distinguished responses that were age related or associated with the clinical course. Our studies suggest that early immune responses mediated by cells producing IL-17A and IFN-γ resulted in more rapid resolution of the viral infection and may have mitigated against the progressive cytokine release and tissue pathology that occurs with more robust adaptive immune responses.
Reference & source information: https://stm.sciencemag.org/
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