Characterization of the immune responses to SARS-CoV-2 reveals a CD8 ⁺ T cell subset that likely establishes the memory to the virus.

The development of a tractable small animal model faithfully reproducing human coronavirus disease 2019 pathogenesis would arguably meet a pressing need in biomedical research. Thus far, most investigators have used transgenic mice expressing the human ACE2 in epithelial cells (K18-hACE2 transgenic mice) that are intranasally instilled with a liquid severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) suspension under deep anesthesia. Unfortunately, this experimental approach results in disproportionate high central nervous system infection leading to fatal encephalitis, which is rarely observed in humans and severely limits this model’s usefulness. Here, we describe the use of an inhalation tower system that allows exposure of unanesthetized mice to aerosolized virus under controlled conditions. Aerosol exposure of K18-hACE2 transgenic mice to SARS-CoV-2 resulted in robust viral replication in the respiratory tract, anosmia, and airway obstruction but did not lead to fatal viral neuroinvasion. When compared with intranasal inoculation, aerosol infection resulted in a more pronounced lung pathology including increased immune infiltration, fibrin deposition, and a transcriptional signature comparable to that observed in SARS-CoV-2–infected patients. This model may prove useful for studies of viral transmission, disease pathogenesis (including long-term consequences of SARS-CoV-2 infection), and therapeutic interventions.

Vaccines have reduced severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) morbidity and mortality, yet emerging variants challenge their effectiveness. The prevailing approach to updating vaccines targets the antibody response, operating under the presumption that it is the primary defense mechanism following vaccination or infection. This perspective, however, can overlook the role of T cells, particularly when antibody levels are low or absent. Here we show, through studies in mouse models lacking antibodies but maintaining functional B cells and lymphoid organs, that immunity conferred by prior infection or mRNA vaccination can protect against SARS-CoV-2 challenge independently of antibodies. Our findings, using three distinct models inclusive of a novel human/mouse ACE2 hybrid, highlight that CD8⁺ T cells are essential for combating severe infections, whereas CD4⁺ T cells contribute to managing milder cases, with interferon-γ having an important function in this antibody-independent defense. These findings highlight the importance of T cell responses in vaccine development, urging a broader perspective on protective immunity beyond just antibodies.

Vaccines against COVID-19 are a powerful tool to control the current SARS-CoV-2 pandemic. A thorough description of their immunogenicity among people living with HIV (PLWHIV) is necessary. We aimed to assess the immunogenicity of the mRNA-1273 vaccine among PLWHIV. In this prospective cohort, adult PLWHIV outpatients were enrolled during the Italian vaccination campaign. Enrolment was allowed irrespective of ongoing combination antiretroviral therapy (ART), plasma HIV viral load and CD4+ T cell count. A two-dose regimen of mRNA-1273, with administrations performed 28 days apart, was employed. The primary outcomes were anti-spike (anti-S) antibody titres and neutralising antibody activity, assessed 28 days after completing the vaccination schedule. A convenient sample of individuals not affected by HIV was also collected to serve as control (referred as healthy-donors, HDs). We enrolled 71 PLWHIV, mostly male (84·5%), with a mean age of 47 years, a median CD4+ T cell count of 747·0 cells per µL and a median HIV viral load <50 copies/mL. COVID-19-experienced PLWHIV displayed higher anti-S antibody titres (p=0·0007) and neutralising antibody activity in sera (p=0·0007) than COVID-19-naïve PLWHIV. When stratified according to CD4+ T cell count (500 cells/μL), anti-S antibody titres (6/71, median 2173 U/mL [IQR 987-4109]; 7/71, 5763 IU/mL [IQR 4801->12500]; 58/71, 2449 U/mL [IQR 1524-5704]) were not lower to those observed among HDs (10, median 1425 U/mL [IQR 599-6131]). In addition, neutralising antibody activity, stratified according to the CD4+ T cell count (6/71, median 1314 [IQR 606-2477]; 7/71, 3329 IU/mL [IQR 1905-10508]; 58/71, 1227 U/mL [IQR 761-3032]), was like those displayed by HDs (10, median 2112 U/mL [IQR 719-8889]). In our cohort of PLWHIV with well-controlled ART, stable viral suppression and robust CD4+ T cell count, inoculation with mRNA-1273 vaccine given 4 weeks apart produced detectable humoral immune response, similar to individuals without HIV infection, supporting vaccination in PLWHIV. This study was partially supported by Italian Ministry of Health Ricerca Corrente 2021, by Intesa San Paolo COVID-19 emergency 2020 funds, and by Fondazione Cariplo Grant (INNATE-CoV).

Significance: Clinical candidate monoclonal antibody J08 binds the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) S-protein independent of known escape mutations and is able to potently neutralize most variants of concern (VoCs). Here, we explore these properties using cell-based assays and structural studies. A relatively small epitope footprint high on the receptor binding domain (RBD) ridge and the ability to bind multiple conformational states of the S-protein contribute to strong neutralization across several variants.

The continuous evolution of SARS-CoV-2 generated highly mutated variants able to escape natural and vaccine-induced primary immunity. The administration of a third mRNA vaccine dose induces a secondary response with increased protection. Here we investigate the longitudinal evolution of the neutralizing antibody response in four donors after three mRNA doses at single-cell level. We sorted 4100 spike protein specific memory B cells identifying 350 neutralizing antibodies. The third dose increases the antibody neutralization potency and breadth against all SARS-CoV-2 variants as observed with hybrid immunity. However, the B cell repertoire generating this response is different. The increases of neutralizing antibody responses is largely due to the expansion of B cell germlines poorly represented after two doses, and the reduction of germlines predominant after primary immunization. Our data show that different immunization regimens induce specific molecular signatures which should be considered while designing new vaccines and immunization strategies.