The global rollout of COVID-19 vaccines has played a critical role in reducing pandemic spread, disease severity, hospitalizations, and deaths. However, the first-generation vaccines failed to block severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and transmission, partially due to the limited induction of mucosal immunity, leading to the continuous emergence of variants of concern (VOC) and breakthrough infections. To meet the challenges from VOC, limited durability, and lack of mucosal immune response of first-generation vaccines, novel approaches are being investigated. Herein, we have discussed the current knowledge pertaining to natural and vaccine-induced immunity, and the role of the mucosal immune response in controlling SARS-CoV2 infection. We have also presented the current status of the novel approaches aimed at eliciting both mucosal and systemic immunity. Finally, we have presented a novel adjuvant-free approach to elicit effective mucosal immunity against SARS-CoV-2, which lacks the safety concerns associated with live-attenuated vaccine platforms.

Francisella tularensis (Ft) is a biothreat agent for which there is no FDA-approved human vaccine. Currently, there are substantial efforts underway to develop both vaccines and the tools to assess these vaccines. Tularemia laboratory research has historically relied primarily upon a small number of inbred mouse strains, but the utility of such findings to outbred animals may be limited. Specifically, C57BL/6 mice are more susceptible than BALB/c mice to Ft infection and less easily protected against challenge with highly virulent type A Ft. Thus, depending on the inbred mouse strain used, one could be misled as to which immunogen(s)/vaccine will ultimately be effective in an outbred human population. Accordingly, we evaluated an outbred Swiss Webster (SW) mouse model in direct comparison to a well-established, inbred C57BL/6 mouse model. Mucosal vaccination with the live, attenuated Ft LVS superoxide dismutase (sodB) mutant demonstrated significantly higher protection in outbred SW mice compared to inbred C57BL/6 mice against Ft SchuS4 respiratory challenge. The protection observed in vaccinated outbred mice correlated with lower bacterial density, reduced tissue inflammation, and reduced levels of pro-inflammatory cytokine production. This protection was CD4⁺ and CD8⁺ T cell-dependent and characterized by lower titers of serum antibody (Ab) that qualitatively differed from vaccinated inbred mice. Enhanced protection of vaccinated outbred mice correlated with early and robust production of IFN-γ and IL-17A. Neutralizing Ab administered at the time of challenge revealed that IFN-γ was central to this protection, while IL-17A neutralization did not alter bacterial burden or survival. The present study demonstrates the utility of the outbred mouse as an alternative vaccination model for testing tularemia vaccines. Given the limited MHC repertoire in inbred mice, this outbred model is more analogous to the human in terms of immunological diversity.

Lack of safe and effective mucosal adjuvants has severely hampered the development of mucosal subunit vaccines. In this regard, we have previously shown that immunogenicity of vaccine antigens can be improved by targeting the antigens to the antigen-presenting cells. Specifically, groups of mice immunized intranasally with a fusion protein (Bivalent-FP) containing a fragment of pneumococcal-surface-protein-A (PspA) as antigen and a single-chain bivalent antibody raised against the anti-human Fc-gamma-receptor-I (hFcγRI) elicited protective immunity to pulmonary Streptococcus pneumoniae infection. In order to further enhance the immunogenicity, an additional hFcγRI-binding moiety of the single chain antibody was incorporated. The modified vaccine (Trivalent-FP) induced significantly improved protection against lethal pulmonary S. pneumoniae challenge compared to Bivalent-FP. In addition, the modified vaccine exhibited over 85% protection with only two immunizations. Trivalent-FP also induced S. pneumoniae-specific systemic and mucosal antibodies. Moreover, Trivalent-FP also induced IL-17- and IL-22-producing CD4⁺ T cells. Furthermore, it was found that the hFcγRI facilitated uptake and presentation of Trivalent-FP. In addition, Trivalent-FP also induced IL-1α, MIP-1α, and TNF-α; modulated recruitment of dendritic cells and macrophages; and induced CD80/86 and MHC-II expression on antigen presenting cells.

The aim of this study was to compare the performance of serological versus molecular typing methods to detect capsular polysaccharide (CP) and surface-associated polysaccharide antigen 336 phenotypes of Staphylococcus aureus isolates. Molecular typing of CP types 1, 5 and 8 was carried out using PCR, whereas serological typing of CP1, 2, 5, 8 and antigen 336 was carried out by slide agglutination using specific antisera. By genotyping, 14/31 strains were CP8 positive, 12/31 strains were CP5 and the remaining 6/31 isolates were non-typable (NT). One isolate was positive for both CP5 and CP8 by PCR, but was confirmed as CP8 type serologically. Detection of CP2 and type 336 by PCR was not possible because specific primers were either not available or non-specific. Using serotyping, 14/31 strains were CP8 positive, 11/31 CP5 positive and 2/31 positive for antigen 336. The remaining four S. aureus isolates were serologically NT. However, three of four NT and two 336-positive S. aureus isolates were encapsulated as determined by light microscopy after capsular staining. This discovery was surprising and warrants further investigations on the identification and characterization of additional capsular phenotypes prevalent among S. aureus clinical isolates. It was concluded that serological typing was a better method than molecular typing for use in epidemiological investigations based upon the distribution of surface-associated polysaccharide antigens-based phenotypes.