Svoboda et al.
A combination of two resistance mechanisms is critical for tick-borne encephalitis virus escape of broadly neutralizing human antibodies.
Cell Rep. 2023;42(9):113149. doi:10.1016/j.celrep.2023.113149.

There is yet no specific therapy for people with TBE, and therefore, development of medical countermeasures against TBE are needed. Monoclonal antibodies (mabs) represent a promising approach against TBE, and the main target of neutralizing mabs is the glycoprotein E with the three structural domains EDI, EDII and EDIII. Potent mabs recognizing the EDIII target have been derived from people who had suffered from TBE, and some of these mabs can also neutralize other flaviviruses than TBE virus and have been shown to be effective in challenge models in mice. Here, the two human mabs T025 and T028 were investigated in detail.

When TBE virus strain Hypr was passaged in vitro in presence of mab T025 in sub-neutralizing concentrations, an escape mutant (TBEV-MUT25) emerged with two nucleotide changes in the EDII and EDIII domains (G1660A and G1905T) resulting in amino acid changes (E230K and K311N). A similar escape mutant (TBEV-MUT28) was performed for mab T028.

These two mutants were stable when passaged in porcine kidney (PS) cells (10 passages) without selective pressure. The plaque morphology in PS cells was markedly different form the parent strain Hypr having smaller plaques. BALBc mice, infected with these mutants, had a significantly lower mortality and longer mean survival, showing that the mutants were less virulent. The E sequence remained unchanged from brains of infected mice.

Experiments to evaluate the role of each mutation revealed that both mutations are required for full escape from the respective antibody.

When the strain Hypr was passaged in sub-neutralizing concentrations of both mabs, no increase in resistance and changes in the E protein were observed, suggesting that the combination of these antibodies may be resistant to escape.

The T025/T028 cocktail revealed significant improved neutralization capacity over the two antibodies alone.

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