NHBA is processed by kallikrein from human saliva.

Neisserial Heparin Binding Antigen (NHBA) is a surface-exposed lipoprotein of Neisseria meningitidis and a component of the Bexsero vaccine. NHBA is characterized by the presence of a highly conserved Arg-rich region involved in binding to heparin and heparan sulphate proteoglycans present on the surface of host epithelial cells, suggesting a possible role of NHBA during N. meningitidis colonization. NHBA has been shown to be cleaved by the meningococcal protease NalP and by human lactoferrin (hLF), a host protease presents in different body fluids (saliva, breast milk and serum). Cleavage occurs upstream or downstream the Arg-rich region. Since the human nasopharynx is the only known reservoir of infection, we further investigated the susceptibility of NHBA to human proteases present in the saliva to assess whether proteolytic cleavage could happen during the initial steps of colonization. Here we show that human saliva proteolytically cleaves NHBA, and identified human kallikrein 1 (hK1), a serine protease, as responsible for this cleavage. Kallikrein-related peptidases (KLKs) have a distinct domain structure and exist as a family of 15 genes which are differentially expressed in many tissues and in the central nervous system. They are present in plasma, lymph, urine, saliva, pancreatic juices, and other body fluids where they catalyze the proteolysis of several human proteins. Here we report the characterization of NHBA cleavage by the tissue kallikrein, expressed in saliva and the identification of the cleavage site on NHBA both, as recombinant protein or as native protein, when expressed on live bacteria. Overall, these findings provide new insights on NHBA as target of host proteases, highlights thepotential role of NHBA in the Neisseria meningitidis nasopharyngeal colonization, and of kallikrein as a defensive agent against meningococcal infection.
Competing Interests: Sara Marchi, Massimiliano Biagini, Vincenzo Nardi-Dei, Silvia Rossi Paccani, Mariagrazia Pizza and Elena Cartocci were employees of Novartis Vaccines at the time of the study; in March 2015 the Novartis non-influenza Vaccines business was acquired by the GSK group of companies. Martina Di Fede was a PhD Student of the University of Siena at the time of the study and supervised by Novartis Vaccines and Diagnostics Srl (and then by GSK). Martina Di Fede is now an employee at Institute für Virologie und Medizinische Biochemie, Westfälische Wilhelms-Universität Münster, Von-Esmarch-Str. 56, 48149, Münster, Germany. Elisa Pantano was a student of the University of Perugia at the time of the study (Internship) supervised by Novartis Vaccines and Diagnostic Srl (and then by GSK). The declared conflicts of interest do not alter authors adherence to PLOS ONE policies on sharing data and materials, and there are no patents, products in development or marketed products to declare.