Loss in CD4 T-cell responses to multiple epitopes in influenza due to expression of one additional MHC class II molecule in the host

CD4 T cell epitope abundance in ferritin core potentiates responses to hemagglutinin nanoparticle vaccines

Nanoparticle vaccines based on H. pylori ferritin are increasingly used as a vaccine platform for many pathogens, including RSV, influenza, and SARS-CoV-2. They have been found to elicit enhanced, long-lived B cell responses. The basis for improved efficacy of ferritin nanoparticle vaccines remains unresolved, including whether recruitment of CD4 T cells specific for the ferritin component of these vaccines contributes to cognate help in the B cell response. Using influenza HA-ferritin nanoparticles as a prototype, we have performed an unbiased assessment of the CD4 T cell epitope composition of the ferritin particles relative to that contributed by influenza HA using mouse models that express distinct constellations of MHC class II molecules. The role that these CD4 T cells play in the B cell responses was assessed by quantifying follicular helper cells (TFH), germinal center (GC) B cells, and antibody secreting cells. When mice were immunized with equimolar quantities of soluble HA-trimers and HA-Fe nanoparticles, HA-nanoparticle immunized mice had an increased overall abundance of TFH that were found to be largely ferritin-specific. HA-nanoparticle immunized mice had an increased abundance of HA-specific isotype-switched GC B cells and HA-specific antibody secreting cells (ASCs) relative to mice immunized with soluble HA-trimers. Further, there was a strong, positive correlation between CD4 TFH abundance and GC B cell abundance. Thus, availability of helper CD4 T cell epitopes may be a key additional mechanism that underlies the enhanced immunogenicity of ferritin-based HA-Fe-nanoparticle vaccines.

Peptide Epitope Hot Spots of CD4 T Cell Recognition Within Influenza Hemagglutinin During the Primary Response to Infection

Antibodies specific for the hemagglutinin (HA) protein of influenza virus are critical for protective immunity to infection. Our studies show that CD4 T cells specific for epitopes derived from HA are the most effective in providing help for the HA-specific B cell responses to infection and vaccination. In this study, we asked whether HA epitopes recognized by CD4 T cells in the primary response to infection are equally distributed across the HA protein or if certain segments are enriched in CD4 T cell epitopes. Mice that collectively expressed eight alternative MHC (Major Histocompatibility Complex) class II molecules, that would each have different peptide binding specificities, were infected with an H1N1 influenza virus. CD4 T cell peptide epitope specificities were identified by cytokine EliSpots. These studies revealed that the HA-specific CD4 T cell epitopes cluster in two distinct regions of HA and that some segments of HA are completely devoid of CD4 T cell epitopes. When located on the HA structure, it appears that the regions that most poorly recruit CD4 T cells are sequestered within the interior of the HA trimer, perhaps inaccessible to the proteolytic machinery inside the endosomal compartments of antigen presenting cells.

Protein Vaccination Directs the CD4+ T Cell Response toward Shared Protective Epitopes That Can Be Recalled after Influenza Virus Infection

Vaccination is widely used to generate protective immunity against influenza virus. CD4⁺ T cells contribute in diverse ways to protective immunity, most notably, in the provision of help for the production of neutralizing antibodies. Several recent reports have suggested that influenza virus infection elicits CD4⁺ T cells whose specificity only partially overlaps that of T cells elicited by vaccination. This finding has raised serious concerns regarding the utility of currently licensed inactivated influenza virus vaccines and novel protein-based vaccines. Here, using controlled animal models that allowed a broad sampling of the CD4⁺ T cell repertoire, we evaluated protein vaccine- versus infection-generated CD4⁺ T cell epitopes. Our studies revealed that all the infection-elicited CD4⁺ T cell epitope specificities are also elicited by protein vaccination, although the immunodominance hierarchies can differ. Finally, using a reverse-engineered influenza virus and a heterologous protein vaccination and infection challenge strategy, we show that protein vaccine-elicited CD4⁺ memory T cells are recalled and boosted after infection and provide early help to accelerate hemagglutinin (HA)-specific antibody responses. The early CD4⁺ T cell response and HA-specific antibody production are associated with lowered viral titers during the infection challenge. Our data lend confidence to the ability of current protein-based vaccines to elicit influenza virus-specific CD4⁺ T cells that can potentiate protective immunity upon influenza virus infection.IMPORTANCE Most current and new influenza vaccine candidates consist of a single influenza virus protein or combinations of influenza virus proteins. For these vaccines to elicit CD4⁺ T cells that can be recalled after infection, the peptide epitopes should be shared between the two modes of confrontation. Recently, questions regarding the relatedness of epitope selection by influenza virus infection and protein vaccination have been raised. However, the studies reported here show that the specificity of CD4⁺ T cells elicited by protein-based vaccines overlaps that of T cells elicited by infection and that CD4⁺ T cells primed by protein vaccines are recalled and contribute to protection of the host from a future infection.

Both positive and negative effects on immune responses by expression of a second class II MHC molecule

It is perplexing why vertebrates express a limited number of major histocompatibility complex (MHC) molecules when theoretically, having a greater repertoire of MHC molecules would increase the number of epitopes presented, thereby enhancing thymic selection and T cell response to pathogens. It is possible that any positive effects would either be neutralized or outweighed by negative selection restricting the T cell repertoire. We hypothesize that the limit on MHC number is due to negative consequences arising from expressing additional MHC. We compared T cell responses between B6 mice (I-A(+)) and B6.E(+) mice (I-A(+), I-E(+)), the latter expressing a second class II MHC molecule, I-E(b), due to a monomorphic Eα(k) transgene that pairs with the endogenous I-Eβ(b) chain. First, the naive T cell Vβ repertoire was altered in B6.E(+) thymi and spleens, potentially mediating different outcomes in T cell reactivity. Although the B6 and B6.E(+) responses to hen egg-white lysozyme (HEL) protein immunization remained similar, other immune models yielded differences. For viral infection, the quality of the T cell response was subtly altered, with diminished production of certain cytokines by B6.E(+) CD4(+) T cells. In alloreactivity, the B6.E(+) T cell response was significantly dampened. Finally, we observed markedly enhanced susceptibility to experimental autoimmune encephalomyelitis (EAE) in B6.E(+) mice. This correlated with decreased percentages of nTreg cells, supporting the concept of Tregs exhibiting differential susceptibility to negative selection. Altogether, our data suggest that expressing an additional class II MHC can produce diverse effects, with more severe autoimmunity providing a compelling explanation for limiting the expression of MHC molecules.