While transiting from replication (exponential phase in vitro) to transmission (stationary phase in vitro), L. pneumophila activates an intricate network of regulators such as LetA/S, RpoS, PmrA, CpxR, rsmYZ, CsrA and LqsR [11, 13, 20, 21, 59]. As shown in our results, unlike the stationary-phase wild type which exhibits transmission traits, LpΔclpP mutant cells in stationary phase
exhibit replicative forms such as reduced stress tolerance (Figure 2 and 3), cell elongation (Figure LY3023414 clinical trial 4), enhanced sodium resistance (Figure 5), impaired cytotoxicity and growth on amoebae plates (Figure 6) and severely compromised intracellular multiplication in amoebae host (Figure 7). Thus, ClpP may play an important role in the transition from replication to transmission in L. pneumophila. On the other hand, several transmission traits are not affected by clpP-deletion such as pigment accumulation and transcription from the flaA (legionella flagellin coding) gene (our unpublished data), suggesting that the impact of ClpP on the transition to transmissive form in L. pneumophila is somewhat limited. Considering that ClpP always executes the post-transcriptional feedback regulation, and
moreover, degrades the same substrates by cooperating with other proteases [26, 31], one explanation to such a limitation is that the degradation of ClpP substrates could be compensated by other proteases in mTOR inhibitor clpP-deletion mutant, thus ClpP cannot govern the transition just as the global regulators such as RpoS, CsrA or LetA/S in L. pneumophila. ClpP plays prominent roles
in virulence of various Gram-positive pathogens such as S. aureus, S. pneumoniae and L. monocytogenes [34–36, 60]. Furthermore, ClpP was reported to control the levels of key virulence factors of type III secretory systems (T3SS) in certain pathogens such as S. typhimurium and Yersinia pestis [61, 62]. Recently, it was reported Palmatine that loss of ClpP attenuated the virulence of Helicobacter pylori, a pathogen owning type IV secretory system (T4SS) . It is interesting that clpP-deletion severely compromised the L. pneumophila infection against amoebae host (Figure 6 and 7). In our results, the sodium resistance exhibited by LpΔclpP mutant (Figure 5), which is a phenotype shared by the mutants without functional Dot/Icm T4SS [48, 64], together with the comparable decline in intracellular multiplication observed in LpΔclpP and ΔdotA mutants (Figure 7), suggest a role of ClpP in T4SS-dependent virulence through degrading a repressor or activating an up-regulator of the substrate(s) of ClpP. One possibility is that the ClpP Torin 2 protease has a major impact on the expression or function of Dot/Icm T4SS in L. pneumophila. Another possibility is that ClpP might be required for the expression of some T4SS substrates.