MULTIVALENT PNEUMOCOCCAL CAPSULAR POLYSACCHARIDE CONJUGATE VACCINES EMPLOYING GENETICALLY DETOXIFIED PNEUMOLYSIN AS A CARRIER PROTEIN

A genetically detoxified pneumolysin, pneumolysoid (PLD), was investig ated as a carrier protein for pneumococcal capsular polysaccharide (CP S). Such a CPS-PLD conjugate might provide additional protection again st pneumococcal infections and resultant tissue damage. A single point mutant of pneumolysin was selected, which lacked measurable haemolyti c activity, but exhibited the overall structural and immunological pro perties of the wild type. PLD conjugates were prepared from CPS seroty pes 6B, 14, 19F and 23F by reductive amination. The structural feature s of free PLD, as well as the corresponding CPS-PLD, as assessed by ci rcular dichroism spectroscopy, were virtually indistinguishable from t he wild type counterpart Each of the CPS monovalent and tetravalent co njugate formulations were examined for immunogenicity in mice at both 0.5 and 2.0 mu g CPS per dose. Tetanus toroid (TT) conjugates were sim ilarly created and used for comparison. The resultant conjugate vaccin es elicited high levels of CPS-specific IgG that was opsonophagocytic for all serotypes tested. Opsonophagocytic titres, expressed as recipr ocal dilutions resulting in 50% killing using HL-60 cells, ranged from 100 to 30000, depending on the serotype and formulation. In general, the lower dose and tetravalent formulations yielded the best responses for all serotypes (i.e., either equivalent or better than the higher dose and monovalent formulations). The PLD conjugates were also genera lly equivalent to or better in CPS-specific responses than the TT conj ugates. In particular both the PLD conjugate and the tetravalent formu lations induced responses for type 23F CPS that were approximately an order of magnitude greater than that of the corresponding TT conjugate and monovalent formulations. In addition, all the PLD conjugates elic ited high levels of pneumolysin-specific IgG which were shown to neutr alize pneumolysin-induced haemolytic activity in vitro. As a result of these findings, PLD appears to provide an advantageous alternative to conventional carrier proteins for pneumococcal multivalent CPS conjug ate vaccines. (C) 1998 Published by Elsevier Science Ltd. All rights r eserved.