Exploiting vita-PAMPs in vaccines

Abstract

Live attenuated vaccines elicit stronger protective immunity than dead vaccines. Distinct PAMPs designated as vita-PAMPs signify microbial viability to innate immune cells. Two vita-PAMPs have been characterized: cyclic-di-adenosine-monophosphate (c-di-AMP) and prokaryotic messenger RNA (mRNA). c-di-AMP produced by live Gram-positive bacteria elicits augmented production of STING-dependent type-I interferon, whereas prokaryotic mRNA from live bacteria is detected by TLR8 enabling discrimination of live from dead bacteria. Bacterial mRNA from live Gram-negative bacteria triggers a heightened type-I interferon and NLRP3 inflammasome response. By mobilizing unique viability-associated innate responses, vita-PAMPs mobilize adaptive immunity that best elicits protection, including follicular T helper cell and antibody responses. Here, we review the molecular mechanisms that confer the unique adjuvanticity of vita-PAMPs and discuss their applications in vaccine design.

Figure 1. The molecular and cellular events subsequent to vita-PAMP recognition

In mice, mononuclear phagocytes respond to microbial viability. Live Gram-positive and Gram-negative bacteria contain vita-PAMPs, such as bacterial mRNA and c-di-AMP, respectively, which serve as the molecular signatures of bacterial viability. Both vita-PAMPs induce type-I interferon through the Toll-like receptor adaptor TRIF for Gram-negative bacteria and through the stimulator of interferon genes (STING) and the kinase TBK1 for Gram-positive bacteria. In the response to phagocytosed Gram-negative bacteria, bacterial mRNA which gains access to the cytosol activates the NLRP3 inflammasome (1a) and IFNAR activation by TRIF-dependent IFN-β (1b) licenses NLRP3 inflammasome activation by promoting Caspase-11 expression (2) and interaction with caspase-1 to mediate IL-1β cleavage and secretion (3). In the response to phagocytosed Gram-positive bacteria, c-di-AMP gains access to STING on endoplasmic reticulum(ER) membranes to induce ER stress and subsequent ER-stress induced autophagy to orchestrate translocation of STING to autophagosomal membranes, TBK1 activation, and induction of type-I IFN production.

Figure 2. Cellular activation after sensing the vita-PAMP bacterial mRNA

Bacterial mRNA recognition by monocytes increases follicular T helper cell differentiation, enhances IgG antibody response through higher follicular T helper cells and higher germinal center formation for mouse, human, and porcine (not shown) models. In mice, T_FH cell differentiation by a live vaccine comprised of Gram-negative bacteria or a dead counterpart vaccine supplemented with bacterial mRNA is dependent on TRIF signaling and downstream cytokines IFN-β and IL-1β. Both cytokines act directly on mouse T cells to promote full T_FH differentiation. In the human model, activation of the MyD88-dependent TLR8 receptor on monocytes induces the expression of IL-12, the main cytokine promoting human T_FH cell differentiation, and induces T_FH differentiation of co-cultured activated CD4 T cells. Human IL-1β is also produced in response to live bacteria by a yet to be explored inflammasome pathway and has additive effects on T_FH differentiation. Despite the different cytokine requirements for T_FH cell differentiation, detection of bacterial viability by either mouse or human monocytes serves as the physiological trigger for T_FH cell differentiation and subsequent B cell class-switching and IgG production.