Protein kinase F regulates the virulence of Mycobacterium tuberculosis
The serine/threonine protein kinase F (PknF) of Mycobacterium tuberculosis (Mtb) has poorly defined targets and functions but is involved in limiting NLRP3 inflammasome activation in murine macrophages and dendritic cells in vitro. The importance of PknF for the virulence of Mtb in vivo is not known. Here, we demonstrate that the Mtb CDC1551 deletion mutant of pknF ({Delta}pknF) expresses significantly increased levels of the lipid pthiocerol dimycoserosate (PDIM), a polyketide lipid with pro-virulence properties. The {Delta}pknF mutant strain, when compared to the Mtb and complemented strains, had a 100-fold increase in growth at day 28 and about a 10-fold increase in growth at days 90-98 in the lungs of mice. The increase in pulmonary bacterial loads after infection with {Delta}pknF strain was conserved even in Nlrp3-deficient mice, arguing that PknF modulates Mtb virulence independently of NLRP3-inflammasome activation in mice. Staining of lung sections revealed increased inflammation in the lungs of {Delta}pknF strain-infected mice when compared to Mtb and the complemented mutant strain. Highly susceptible B6.Sst1S mice displayed decreased host resistance with significantly decreased survival when infected with the {Delta}pknF strain compared to the complemented strain or Mtb. In conclusion, our data suggest that expression of PknF, as a modulator of multiple downstream effector proteins, restricts the virulence of Mtb in the lungs of mice through an NLRP3 inflammasome-independent mechanism but potentially via suppressing expression of the virulence lipid, PDIM. Author summaryThe human pathogen Mycobacterium tuberculosis (Mtb) encodes 11 serine/threonine protein kinases (Pkn A-I,K and L). In vitro, PknF inhibits activation of innate immune responses in macrophages and dendritic cells. The importance of that protein for the virulence of the bacteria in the context of a live animal infection is unknown. We used a hypersusceptible mouse strain to analyze the impact of deleting the pknF gene on the virulence of the bacteria by monitoring the survival of the mice. We show that the {Delta}pknF deletion mutant kills mice faster than wild-type bacteria or {Delta}pknF mutant bacteria expressing a wild-type copy of the pknF gene ({Delta}pknF-C). Next, we analyzed the growth of the different bacterial strains in the infected mice and demonstrated that at day 28 and day 90-98 timepoints the {Delta}pknF mutant bacterial strains showed a significant increase in bacterial burden in the lungs, bronchoalveolar lavage fluid and the spleen. An analysis of the total lipids of the bacterial stocks showed that the {Delta}pknF Mtb bacteria increased expression of the virulence lipid, pthiocerol dimycoserosate (PDIM), suggesting a negative regulation of this lipid by PknF likely affecting bacterial virulence in the lungs of mice.