Immunization with mRNA-LNP Elicits De Novo IgG Responses in the Presence of Maternal Antibody

Abstract

Background/Objectives: Maternal antibodies can inhibit vaccine-specific humoral responses in early life, leaving infants at increased risk for severe disease for vaccine-preventable infections. In the case of SARS-CoV-2, infants under the age of 3 months represented most child hospitalizations, yet there is no approved vaccine for children under the age of 6 months. There is a clear need for effective immunization in early life to prevent infant morbidity and mortality. Here, we established a mouse model to define how maternally derived antibodies shape early-life responses to mRNA vaccination. Methods: Adult female mice were immunized with PBS or 5 mcg of the SARS-CoV-2 mRNA-1273 vaccine via intramuscular injection and paired with a male. Pups from subsequent litters were immunized with PBS or 5 mcg of mRNA-1273 vaccine via intramuscular injection. Peripheral blood and spleens were collected at time points post-immunization. We measured vaccine-elicited anti-Spike IgG in mouse pups exposed or unexposed to vaccine-specific maternal IgG. Results: Spike-specific maternal IgG is detectable at high levels immediately after pup immunization or mock immunization; however, in mock immunized pups, it wanes by three weeks post-pup immunization. Pups born to immunized dams developed Spike specific IgG comparable to pups born to naïve dams. IgG subclass analyses distinguished passively acquired antibodies from vaccine-induced responses. Despite robust binding antibody responses, neutralizing activity against D614G pseudovirus was heterogeneous and did not scale proportionally with IgG titers, showing qualitative differences in early-life humoral immunity. Splenic Spike-specific B cell frequencies and T follicular helper (Tfh) cell responses were detectable in vaccinated pups irrespective of maternal immunization status, with Tfh cell frequencies peaking at day 7 post-immunization in both groups. Conclusions: Using SARS-CoV-2 as a model pathogen, we found that early-life mRNA vaccination can elicit humoral immune responses in the presence of maternal antibodies. Furthermore, the presence of maternal antibody did not inhibit the development of antigen-specific B cells or Tfh cells in the spleen. Our findings support the potential of extending vaccination strategies into early infancy and provide a framework for optimizing mRNA-based vaccine timing and design in the context of maternal immunity.