Stronger T Cell Immunogenicity of Ovalbumin Expressed Intracellularly in Gram-Negative than in Gram-Positive Bacteria

<div><p>This study aimed to clarify whether Gram-positive (G+) and Gram-negative (G−) bacteria affect antigen-presenting cells differently and thereby influence the immunogenicity of proteins they express. Lactobacilli, lactococci and <i>Escherichia coli</i> strains were transformed with plasmids conferring intracellular ovalbumin (OVA) production. Murine splenic antigen presenting cells (APCs) were pulsed with washed and UV-inactivated OVA-producing bacteria, control bacteria, or soluble OVA. The ability of the APCs to activate OVA-specific DO11.10 CD4⁺ T cells was assessed by measurments of T cell proliferation and cytokine (IFN-γ, IL-13, IL-17, IL-10) production. OVA expressed within <i>E. coli</i> was strongly immunogenic, since 500 times higher concentrations of soluble OVA were needed to achieve a similar level of OVA-specific T cell proliferation. Furthermore, T cells responding to soluble OVA produced mainly IL-13, while T cells responding to <i>E. coli</i>-expressed OVA produced high levels of both IFN-γ and IL-13. Compared to <i>E. coli</i>, G+ lactobacilli and lactococci were poor inducers of OVA-specific T cell proliferation and cytokine production, despite efficient intracellular expression and production of OVA and despite being efficiently phagocytosed. These results demonstrate a pronounced difference in immunogenicity of intracellular antigens in G+ and G− bacteria and may be relevant for the use of bacterial carriers in vaccine development.</p></div