Structural and functional features of Klebsiella pneumoniae capsular degradation by the phage depolymerase KP32gp38: Implications for vaccination against K. pneumoniae

Objective: Klebsiella pneumoniae Przondovirus KP32 presents a complex capsular degradation machinery comprised of two serotype-specific depolymerases, KP32gp38 and KP32gp37. Methods: In this work, we performed capsular polysaccharide (CPS) degradation assays combined with mass spectrometry approaches to identify the reaction product of K21 serotype CPS degradation by KP32gp38. We determined the crystal structure of the KP32gp38 depolymerase in complex with the identified degradation product, a pyruvated pentasaccharide, called K21-pyr5. Results: The structure showed that K21-pyr5 binds to the inter-chain catalytic site, allowing the identification of important residues for CPS recognition. Importantly, we observed that the production of K21-pyr5 through CPS degradation by KP32gp38 is able to induce the maturation and differentiation of monocyte-derived dendritic cells, which, in turn, induce lymphocyte proliferation and Th polarization. By employing a T7 phage of Escherichia coli analogy, we were able to provide insights into the portal assembly of the Przondovirus K32. Our modeling studies suggest that the KP32 portal, attached to its icosahedral capsid shell, carries 12 depolymerase molecules on a single virion, arranged in 6 branches; in each branch, KP32gp38 depolymerase adheres to KP32gp37, which is directly connected to the phage portal. Conclusions: Overall, our results suggest that depolymerases act as anti-virulent agents, not only by depleting the bacteria of their CPS but also by producing immunostimulatory CPS degradation products. This indicates the use of CPS degradation products by depolymerases as potential antigens in K. pneumoniae vaccination strategies.