Studies on the mechanism of antigen targeting to dendritic cells and the regulation of dendritic cell functions

Abstract

Dendritic cells (DCs) are the most important antigen presenting cells (APCs) for the immune induction, playing a central role in both innate and adaptive immunity. As sentinels, DCs continuously sample self or non-self antigens and harness the environmental stimuli to maintain homeostasis of our immune system. Upon recognition of ‘danger signals’ derived from invading pathogens, DCs undergo functional changes, called maturation or activation, which are specialized for antigen presentation to immune cells in adaptive immune system. Antigen targeting or delivery to DCs and induction of DC maturation are two important key processes for the induction of immune responses. The understanding of these processes in immune induction is important for the development of successful vaccine or therapeutic regimens against microbial infections as well as tumors. Thus, the strategies to enhance antigen targeting to DCs as well as to increase DC maturation have been extensively studied in vaccine development. First of all, I proposed a novel mechanism for antigen targeting to DCs in genetic vaccination, which mimics natural viral infection in somatic cells but not DCs, and in which antigen targeting is an important consideration to enhance vaccine efficacy. HIV protease (PR) mediates the processing of HIV poly-proteins and is necessary for the viral production. HIV PR was shown to possess both cytotoxic and chaperone-like activity. Here, I found that HIV PR can serve as a genetic adjuvant that enhances HIV - and HPV-DNA vaccine-induced T cell responses in a dose dependent manner, only when co-delivered with DNA vaccine.Interestingly, T cell adjuvant effects of HIV PR were increased by introducing several mutations that inhibited its proteolytic activity, indicating that the adjuvant properties were inversely correlated with its proteolytic activity. Conversely, the introduction of a mutation in the flap region of HIV PR limiting the access to the core domain of HIV PR inhibited the T cell adjuvant effect, suggesting that the chaperone-like activity of HIV PR may play a role in mediating T cell adjuvant properties. Similar adjuvant effects were also observed in adenovirus vaccine, indicating vaccine type independency. These findings suggest that HIV PR can modulate T cell responses elicited by gene-based vaccine positively by inherent chaperone-like activity and negatively by its proteolytic activity. Due to their unique characteristics, DCs were generated ex vivo and evaluated as a therapeutic cell-based vaccine against tumor or chronic infectious disease. In this circumstance, antigen targeting and induction of maturation are less important considerations since antigen can be easily treated and DCs can be properly stimulated ex vivo. Thus, I examined an in vivo mechanism affecting the functions and stimulatory capacity of ex vivo generated DCs. DCs can also interact with CD1d-restricted NKT cells via the CD1d presentation of both exogenous and endogenous glycolipid or the secretion of IL-12 or IL-18 even in the absence of CD1d presentation. Thus, I evaluated the effect of type I and II NKT cells on DC induced CD8 T cell responses especially in the absence of exogenous ligands. CD1d-/- DCs were shown to stimulate CD8 T cells in vivo more efficiently than wild-type (WT) DCs. Conversely, CD1d over-expression inhibited DC induced CD8 T cell responses both in WT and iNKT-/- mice lacking type I NKT cells, suggesting type II NKT cell negatively regulates DC function in CD1d-dependent manner. Moreover, IL-12 produced by DCs affected type I NKT cells subsequently limiting DC-induced CD8 T cell responses. Thus, the absence of type I NKT cells and CD1d molecules of immunized DCs cumulatively enhance DC-induced CD8 T cell responses, showing the differential inhibitory mechanisms depending on NKT cell subsets. Overall, these findings provide insights on the mechanism for antigen targeting to DCs as well as the NKT cell mediated regulation of DC-induced CD8 T cell responses and have an important implication for improving gene or DC-based cell therapy.