Kallikrein 13: a new player in coronaviral infections

Aleksandra Milewska; Katherine Falkowski; Magdalena Kalinska; Ewa Bielecka; Antonina Naskalska; Pawel Mak; Adam Lesner; Marek Ochman; Maciej Urlik; Jan Potempa; Tomasz Kantyka; Krzysztof Pyrc

The experiments performed herein show the importance of KLK13 for virus entry into 307 susceptible cells; therefore, we speculated that scattered distribution of different KLKs in 308 different tissues may be one of the determinants of the HCoV-HKU1 tropism (53, 91). We 309 tested the purified enzyme expressed in the eukaryotic cells; however, we also developed a cell 310 line constitutively expressing the enzyme. As an in vitro model for our studies, we used RD 311 cells previously reported to carry attachment receptors for the virus (26). Here, using 312 pseudoviruses decorated with S-HKU1 proteins we showed that KLK13 presence on RD cells 313 is sufficient for virus entry and renders these cells permissive. Our experiments also showed 314 that in contrast to previous reports, TMPRSS2 is not involved in this process(51). Furthermore, 315 we observed that RD cells supported the replication of the virus in the presence of KLK13 and 316 that this effect was reversed in the presence of the specific KLK13 inhibitor. We were, however, 317 not able to culture the virus to high yields. HCoV-HKU1 replication in KLK13-expressing RD 318 cells remained inefficient and RTqPCR assessment did not reveal significant increases in the 319 amounts of viral RNA. For that reason, we are only able to detect viral sg mRNAs, which are 320 considered to be the hallmark of coronaviral replication.We believe that this may be due to 321 non-optimal infection conditions, which may include inappropriate KLK13 concentrations or 322 low density of the entry receptor. Also, it is possible that RD cells may not support efficient 323 replication of the virus due to factors unrelated to the entry process. Nonetheless, our results