Vaccine adjuvants to engage the cross-presentation pathway

Abstract

Adjuvants are indispensable components of vaccines for stimulating optimal immune responses to non-replicating, inactivated and subunit antigens. Eliciting balanced humoral and T cell-mediated immunity is paramount to defend against diseases caused by complex intracellular pathogens, such as tuberculosis, malaria, and AIDS. However, currently used vaccines elicit strong antibody responses, but poorly stimulate CD8 cytotoxic T lymphocyte (CTL) responses. To elicit potent CTL memory, vaccines need to engage the cross-presentation pathway, and this requirement has been a crucial bottleneck in the development of subunit vaccines that engender effective T cell immunity. In this review, we focus on recent insights into DC cross-presentation and the extent to which clinically relevant vaccine adjuvants, such as aluminum-based nanoparticles, water-in oil emulsion (MF59) adjuvants, saponin-based adjuvants, and Toll-like receptor (TLR) ligands modulate DC cross-presentation efficiency. Further, we discuss the feasibility of using carbomer-based adjuvants as next generation of adjuvant platforms to elicit balanced antibody- and T-cell based immunity. Understanding of the molecular mechanism of DC cross-presentation and the mode of action of adjuvants will pave the way for rational design of vaccines for infectious diseases and cancer that require balanced antibody- and T cell-based immunity.

Keywords: CD8 T cells; adjuvants; cross-presentation; immunity; memory; vaccines.

Figure 1

Schematic Overview of the Vacuolar Pathway of Cross-presentation. The internalized antigens are processed by residential cathepsin (cathepsin S; Cat S), Insulin regulated aminopeptidase (IRAP), or proteasomes in the Rab14⁺ and IRAP⁺ phagosomes. The processed peptides are loaded onto MHC-I molecules derived from cellular membrane. The peptide-MHC I complexes (pMHC-I) are trafficked back to the plasma membrane by recycling endosomes, which are mediated by kinesin-1 and microtubules.

Figure 2

Schematic Overview of the Cytosolic Pathway of Cross-presentation. Antigens are internalized into the alkaline phagosomes by a process that is tightly regulated by various proteins, such as NADPH oxidases 2 (NOX2), Aquaporin 3(AQP3), and Rab39a. NOX2 complexes are recruited to the phagosomes by lysosome-related organelle (LRO) mediated by Rab27a and Rac Family Small GTPase 2 (Rac2). ER-Golgi intermediate compartment (ERGIC) derived from endoplasmic reticulum (ER) deliver various intracellular components required for cytosolic pathway, such as Rab39a, Transporter Associated With Antigen Processing (TAP), and SEC22 Homolog B 61(Sec61), which is coordinated by sec22b. The antigens are unfolded by Gamma-interferon-inducible lysosomal thiol reductase (GILT), which will be released into cytosol, presumably by leaky membranes effected by sec61, or heat shock protein (HSP90). Partially unfolded peptides will be further processed by cytosolic proteasomes and transported back to the phagosomes by TAP. The antigenic peptides are processed by IRAP, so that they can be readily loaded onto MHC-I molecules to form pMHC-I molecules are either derived from endolysosomes derived from ER, which is mediated by CD74, or recycled from endosomal recycling component (ERC) from plasma membrane.

Figure 3

Mechanisms of Adjuvant-mediated Cross-presentation in DCs. I. Alum-based nanoparticles: Antigens that are coupled with Alum-based nanoparticles are taken up by scavenger receptor A. Antigens are translocated from phagosomes-to-cytosol, further processed by cytosolic proteasomes, and loaded onto MHC-I molecules by TAP-dependent mechanism. II. Saponin-based adjuvants: Saponin-based adjuvants induce the formation of intracellular lipid bodies (LB). Internalized antigens are localized in the phagosomes, which are released into cytosol facilitated by lysosomal proteases, and degraded by cytosolic proteasomes. The digested peptides are translocated back to ER by TAP transporters and loaded onto MHC I molecules. III. TLR-based adjuvants: Ligation of TLR agonist induces phagocytosis of antigen mediated by Myd88-IKK2-SNAP23 and recruitment of endosomal recycling complex, marked by Rab11a and VAMP 3/8. TLR-based adjuvants also elicit the formation of perinuclear clustering of lysosomes, which leads to delayed fusion of phagolysosomes. IV. Carbomer-based adjuvants: Carbomer-based adjuvants induce the formation of intracellular lipid bodies. Carbomer-based adjuvants also induce the production of ROS in the phagosomes. The internalized antigens escape from phagosomes to cytosol, which are subsequently processed by cytosolic proteasomes, translocated using TAP transporters, and loaded onto MHC-I molecules.