Choice of host model and manipulated transcription regulator dictates T6SS effector-mediated toxicity

Type VI secretion system (T6SS), a toxic effector-delivery apparatus primarily studied for its antibacterial properties, has recently emerged as a widespread anti-eukaryotic determinant, particularly in members of the genus Vibrio. Although various anti-eukaryotic effectors have been described, it remains unknown whether effectors target distinct hosts in a similar manner. Moreover, it is unclear whether the entire effector arsenal encoded within a single genome is co-regulated under different conditions and by different regulators. Here, we employ the anti-eukaryotic T6SS3 of Vibrio proteolyticus to address these knowledge gaps. By monitoring toxicity during infection of oyster hemocytes and comparing the results with previous infections of murine macrophages, we find that the effector Tie1 plays a role only in intoxicating murine cells, whereas Tie2 intoxicates both cell models. Furthermore, we show that artificial induction of T6SS3 via deletion of the negative regulator hns1 fails to induce the orphan effector Tie3, whereas overexpression of a T6SS3-specific activator, Ats3, induces all three known effectors. Ats3 overexpression further reveals that Tie3, not only Tie2, participates in hemocyte intoxication. Collectively, these findings indicate that analyses of T6SS-mediated infections must factor in the possibility of partial effector repertoire activation even when the main gene cluster is fully induced, and that the contribution of anti-eukaryotic T6SS effectors to intoxication is host-dependent.