The diagnosis of an malignant cancer is always life-threatening and goes hand in hand with a long lasting therapy with many and partially severe side effects. To overcome this, the development of new therapeutic approaches is indispensible.
In the last decade, many live-attenuated viruses were tested with different success. Another approach with replication-competent virus was published to be effective in mice. Unfortunately, the used WE-strain of Lymphocytic Choriomeningitis Virus (LCMV) induced transient hepatitis in mice.
In this work, the wildtype virus WE was passaged on human cancer cells in order to shift the viral tropism from healthy cells towards malignant cells. And indeed, two mutations within the tropism-defining glycoprotein were found after 52 passages and characterized in vitro and in vivo.
The in vitro replication on different human tumor cell lines showed a clear pattern. The mutant virus, and to a certain level also the single-mutants carrying only one of the two mutations, yield higher virus titers in the supernatant at early timepoints, especially after infection with low doses of virus. This supports the hypothesis of changed tropism and higher binding affinity to tumor cells.
Infection of naïve mice with the mutated virus induced significantly stronger T cell response compared to WE. In contrast, the serum levels of alanine transaminase (ALT), aspartate transaminase (AST) and lactate dehydrogenase (LDH) that corelate with destruction of virus-infected hepatocytes by virus-specific T cells was reduced in all mutant virus infected mice indicating less infection of hepatocytes. This underlines the tropism change with less infection of healthy cells. Additionally, the peak values of type I interferon (IFN-I) was significantly reduced on day 1 in serum which might reduce the early side effects of the future therapy.
A beneficial role of mutated LCMV was also found in combination with blockade of programmed cell death 1 (PD-1), offering oppertunities for later combination treatments.
Considering the higher replication in tumor cells and decreased infection of healthy cells in combination with reduced side effects, the mutated virus is superior to the wildtype regarding its capacity as viroimmunotherapetic. This examples proofs the possibility to generate a virus with better anti-tumoral properties by simple and fast serial passages. This opens up the chance to further improve the virus or combine mutations to generate a well-working and safe virotherapeutic agent.