Distinct mechanisms of integrin binding by Yersinia pseudotuberculosis adhesins determine the phagocytic response of host macrophages

The enteropathogenic yersiniae express two outer membrane adhesins, invasin and YadA, that contribute to pathogenesis. While invasin binds directly to beta1 integrin receptors with high affinity, YadA binds indirectly through extracellular matrix (ECM) components. In this study, Yersinia pseudotuberculosis inv and yadA mutants were used to investigate how these distinct binding mechanisms compare and potentially compete in activating signalling pathways and promoting bacterial uptake by host macrophages. The efficiency of adhesin-mediated phagocytic responses was found to be dependent on the relative expression of invasin and YadA on the bacterial surface as well as the expression of ECM proteins in the extracellular milieu. Under conditions of low concentrations of ECM, invasin was found to be the dominant adhesin, promoting high levels of phagocytosis coincident with robust and sustained activation of the protein tyrosine kinases Fak and Pyk2, phosphorylation of the adaptor molecule Cas and activation of the small GTPase Rac1. In the presence of higher concentrations of ECM, YadA became the dominant functional adhesin through its ability to engage integrin receptors via an ECM bridge. We propose a model whereby invasin promotes robust and prolonged activation of phagocytic signalling cascades by inducing a 'high-affinity' integrin conformation as well as integrin clustering. We postulate that YadA-ECM promotes phagocytosis through a more transient activation of signalling cascades that arises from integrin clustering in the context of a cross-linked fibrillar ECM network.