Compound 48/80 (c48/80) is a widely studied MC activator and is a potent mucosal adjuvant
[35]. It is a synthetic polymer that stimulates the degranulation of MCs in an IgE-independent manner to evoke an inflammatory response
[54]. Following activation with c48/80, MCs produce TNF-α which promotes DC and T cell functions. leading to significant trafficking of DCs to draining lymph nodes (DLNs)
[37]. Numerous studies have demonstrated the ability of c48/80 to induce high concentrations of serum IgG and mucosal soluble IgA when co-administered intranasally with viral glycoproteins
[55][56][57]. Additionally, c48/80-adjuvanted responses were shown to enhance protection in challenge models using lethal doses of H1N1 influenza virus, as well as vaccinia virus
[56][57]. Interestingly, a follow up study on influenza demonstrated the inability of c48/80 to enhance protection against H1N5 challenge, suggesting that the adjuvanticity of c48/80 may vary depending on the immunogen utilized
[58]. Similarly, the T helper cell responses induced by c48/80 also varied between studies
[55][57]. One study co-administering c48/80 with the hepatitis B virus glycoprotein found a T helper-2-biased response while the studies focusing on the H1N1 influenza found a balanced T helper-1/T helper-2 response
[55][57]. However, it is important to note that the study immunizing with hepatitis B virus glycoprotein utilized a chitosan nanoparticle platform, which may be related to the T helper-2 bias
[55]. While the T helper cell response was not characterized in the study utilizing vaccinia virus, c48/80 induced only a modest increase in the number of CD8
+ T cells, potentially indicating a T helper-2 bias
[56]. In contrast with these studies, one group investigated the ability of c48/80 to enhance CD8
+ T cell-mediated protective immunity utilizing the nucleoprotein of the H1N1 influenza virus
[59]. This group found that c48/80 induced high levels of IgG1 and IgG2a in similar proportions, indicating a potentially well balanced T helper-1/T helper-2 response
[59]. More importantly, it induced a higher number of nucleoprotein-specific CD8
+ T cells compared to CD4
+ T cells. The c48/80+nucleoprotein combination provided 100% protection in mice when challenged with homologous H1N1 influenza virus and even protected against heterologous challenge using the H9N2 strain of influenza virus
[59]. While high concentrations of serum IgG and secretory IgA were produced in these mice, the fact that the nucleoprotein is contained within virus particles and infected cells suggests that the nucleoprotein-specific CD8
+ T cells were the main correlate of protection against virus challenge
[59]. Based on these studies, c48/80 appears to be a potent mucosal adjuvant capable of inducing protective antibody- and cell-mediated immunity against certain viruses. While potentially promising for use as an adjuvant in COVID-19 vaccines, the immunogen-dependent effect of c48/80 and variability in T helper cell responses necessitates extensive investigation.