Exploring of primate models of tick-borne flaviviruses infection for evaluation of vaccines and drugs efficacy.

Tick-borne encephalitis virus (TBEV) is one of the most prevalent and medically important tick-borne arboviruses in Eurasia. There are overlapping foci of two flaviviruses: TBEV and Omsk hemorrhagic fever virus (OHFV) in Russia. Inactivated vaccines exist only against TBE. There are no antiviral drugs for treatment of both diseases. Optimal animal models are necessary to study efficacy of novel vaccines and treatment preparations against TBE and relative flaviviruses. The models for TBE and OHF using subcutaneous inoculation were tested in Cercopithecus aethiops and Macaca fascicularis monkeys with or without prior immunization with inactivated TBE vaccine. No visible clinical signs or severe pathomorphological lesions were observed in any monkey infected with TBEV or OHFV. C. aethiops challenged with OHFV showed massive hemolytic syndrome and thrombocytopenia. Infectious virus or viral RNA was revealed in visceral organs and CNS of C. aethiops infected with both viruses; however, viremia was low. Inactivated TBE vaccines induced high antibody titers against both viruses and expressed booster after challenge. The protective efficacy against TBE was shown by the absence of virus in spleen, lymph nodes and CNS of immunized animals after challenge. Despite the absence of expressed hemolytic syndrome in immunized C. aethiops TBE vaccine did not prevent the reproduction of OHFV in CNS and visceral organs. Subcutaneous inoculation of M. fascicularis with two TBEV strains led to a febrile disease with well expressed viremia, fever, and virus reproduction in spleen, lymph nodes and CNS. The optimal terms for estimation of the viral titers in CNS were defined as 8-16 days post infection. We characterized two animal models similar to humans in their susceptibility to tick-borne flaviviruses and found the most optimal scheme for evaluation of efficacy of preventive and therapeutic preparations. We also identified M. fascicularis to be more susceptible to TBEV than C. aethiops

Image_1_Experimental Evaluation of the Protective Efficacy of Tick-Borne Encephalitis (TBE) Vaccines Based on European and Far-Eastern TBEV Strains in Mice and in Vitro.PDF

<p>Tick-borne encephalitis (TBE), caused by the TBE virus (TBEV), is a serious public health threat in northern Eurasia. Three subtypes of TBEV are distinguished. Inactivated vaccines are available for TBE prophylaxis, and their efficacy to prevent the disease has been demonstrated by years of implication. Nevertheless, rare TBE cases among the vaccinated have been registered. The present study aimed to evaluate the protective efficacy of 4 TBEV vaccines against naturally circulating TBEV variants. For the first time, the protection was evaluated against an extended number of phylogenetically distinct TBEV strains isolated in different years in different territories. The protective effect did not strongly depend on the infectious dose of the challenge virus or the scheme of vaccination. All vaccines induced neutralizing antibodies in protective titers against the TBEV strains used, although the vaccines varied in the spectra of induced antibodies and protective efficacy. The protective efficacy of the vaccines depended on the individual properties of the vaccine strain and the challenge virus, rather than on the subtypes. The neutralization efficiency appeared to be dependent not only on the presence of antibodies to particular epitopes and the amino acid composition of the virion surface but also on the intrinsic properties of the challenge virus E protein structure.</p

Virus titers (log₁₀ PFU/ml) and the presence or absence of viral RNA (+/−) in CNS and visceral organs of <i>C. aethiops</i> monkeys immunized and non-immunized with inactivated vaccine against TBE (EIPVE) on the 14–16 days after challenge with 7.3 log₁₀ PFU of OHFV, strain Nikitina.

<p>0–<1 PFU in 0.4 ml 10% suspension.</p><p>“−”– RT-PCR is negative for viral RNA.</p><p>“+”– RT-PCR is positive for viral RNA.</p

Virus titers in visceral organs and CNS (log₁₀ PFU/ml) of <i>M. fascicularis</i> monkeys s/c inoculated with TBEV Abs-18 or Sof-16 strains at the early terms after infection (8–10 d.p.i.).

<p>0–<1 PFU in 0.2 ml of 10% suspension; s.c. – spinal cord, l.n. – lymph nodes.</p

The complete blood count in <i>C. aethiops</i> monkeys before and at the different terms after s/c challenge with 7.3 log₁₀ PFU of OHFV strain Nikitina.

<p>ND – not determined; aspartate aminotransferase (AST); alanine aminotransferase (ALT).</p

Histological lesions in the brain of monkeys infected with 6.0 log₁₀ PFU of Abs-18 at the late terms after infection (27–28 days).

<p>A and B. Brainstem (<i>truncus cerebri</i>) of <i>M. fascicularis</i> monkey #26: multiple small vasculitis (1), perivascular edema (2), degenerative changes in neurons (3). Magnification: (A) ×100, (B) ×200. C and D. Subcortical region of <i>M. fascicularis</i> monkey #26: small vasculitis (1) and nodules of neuronophagia (2). Magnification: (C) ×100, (D) ×400. Cortex of cerebellum: E. Fall out of small groups of Purkinje cells in <i>M. fascicularis</i> monkey #34 (1). Magnification ×100.; F. Non-infected normal control. Magnification ×200. Staining by Nissle method was used.</p

Histological lesions in the liver and spleen of <i>M. fascicularis</i> monkey (#26) at the late terms after infection with 6.0 log₁₀ PFU of Abs-18.

<p>A. Liver: lymphohistiocytic infiltration of portal liver tracts (1). Magnification ×200. B. Spleen: reduction of lymphoid follicles (1); depletion of white pulp along the trabecular arteries (2). Magnification ×400. Staining with hematoxylin and eosin was used.</p

Viral titers (log₁₀PFU/ml) in CNS and visceral organs of <i>C. aethiops</i> monkeys, immunized twice with TBE inactivated vaccine (FSME-Immun), and in non-immunized monkeys on the 23–26 days after s/c challenge with 6.7 log₁₀ PFU of TBEV, strain Absettarov.

<p>0–<1 PFU in 0.1 ml of 10% suspension.</p