Of mice and not humans: how reliable are animal models for evaluation of herpes CD8(+)-T cell-epitopes-based immunotherapeutic vaccine candidates?

Abstract

Herpes simplex virus type 1 and type 2 (HSV-1 and HSV-2)-specific CD8(+) T cells that reside in sensory ganglia, appear to control recurrent herpetic disease by aborting or reducing spontaneous and sporadic reactivations of latent virus. A reliable animal model is the ultimate key factor to test the efficacy of therapeutic vaccines that boost the level and the quality of sensory ganglia-resident CD8(+) T cells against spontaneous herpes reactivation from sensory neurons, yet its relevance has been often overlooked. Herpes vaccinologists are hesitant about using mouse as a model in pre-clinical development of therapeutic vaccines because they do not adequately mimic spontaneous viral shedding or recurrent symptomatic diseases, as occurs in human. Alternatives to mouse models are rabbits and guinea pigs in which reactivation arise spontaneously with clinical herpetic features relevant to human disease. However, while rabbits and guinea pigs develop spontaneous HSV reactivation and recurrent ocular and genital disease none of them can mount CD8(+) T cell responses specific to Human Leukocyte Antigen- (HLA-)restricted epitopes. In this review, we discuss the advantages and limitations of these animal models and describe a novel "humanized" HLA transgenic rabbit, which shows spontaneous HSV-1 reactivation, recurrent ocular disease and mounts CD8(+) T cell responses to HLA-restricted epitopes. Adequate investments are needed to develop reliable preclinical animal models, such as HLA class I and class II double transgenic rabbits and guinea pigs to balance the ethical and financial concerns associated with the rising number of unsuccessful clinical trials for therapeutic vaccine formulations tested in unreliable mouse models.

Fig. 1. The natural history of genital, oro-facial and ocular herpes infections

Left: (1) While most cases of genital herpes are caused by HSV-2, reports of HSV-1 genital infection are increasing. The virus is transmitted through the genital tract (GT) proliferate locally in this primary site of infection, enters the sensory nerves and travels along nerves to the lumbosacral dorsal root ganglia (sacral ganglia or SG) that innervate the GT, where it establishes a latent infection. (2). Sporadic spontaneous reactivation of HSV-2 from latently infected neurons leads to viral shedding in GT which can cause symptomatic recurrent genital herpes, one of the most prevalent sexually transmitted disease. Right: HSV-1 is transmitted through eyes, lips, mouth and cause orofacial and ocular herpes. (3) Ocular herpes is mainly caused by HSV-1, which infects the cornea and then establishes latency in sensory neurons of the trigeminal ganglia (TG). (4) Sporadic spontaneous reactivation of HSV-1 from latently infected neurons leads to viral shedding in saliva and tears which can cause symptomatic recurrent Herpes Stromal Keratitis (HSK), a blinding corneal disease.

Fig. 2

A simple step-by-step representation showing the route of HSV attack (A), establishment of latency (A), spontaneous reactivation (B) and hormone induced reactivation (C). Panel B shows the “spontaneous reactivation” of HSV from latency and the path of anterograde transport from nerve ganglia to primary site of infection. Panel C shows the hormone induced “artificial reactivation” of HSV from latency and the path of anterograde transport from nerve ganglia to primary site of infection. Spontaneous HSV reactivation in mice is a rare event. Statistically, about 1/100 eyes or less shows “spontaneous reactivation” upon a period of extended sampling. Also <1% of TG have infectious virus but too low for useful analysis. Using RT-PCR 1 neuron/10 TG has lytic transcripts detectable.

Fig. 3

Schematic flow chart representing the HSV infection-reactivation cycle and the potential targeted step where therapeutic vaccine should intervene this cycle. The blue arrow indicates infections, latency, spontaneous reactivation, viral shedding, recurrent infection and eye disease. The brown arrow and the brown cross -indicated inhibition of spontaneous reactivation, viral shedding, recurrent infection and eye disease.

Fig. 4

A schematic representation showing the possible role of asymptomatic CD8⁺ T cells (specific to human HSV-1 CD8⁺ T cell epitope) in decreasing ocular herpes infection and disease in the newly identified HLA-A*0201 transgenic (HLA Tg) rabbit model of spontaneous reactivation, recurrent corneal disease. Unlike what happens in the mouse model, in the rabbit model, similar to humans, circulating HSV-1-specific CD8⁺ T cells do access HSV-1 latently infected trigeminal ganglia where they encounter newly made antigens made following spontaneous reactivation of the virus from latency.