Science commentary: Why conjugate vaccines protect longer

BMJ 1998;316:1571 ( 23 May )

Papers

Science commentary: Why conjugate vaccines protect longer

Abi Berger, Science editor.

BMJ

The effectiveness of polysaccharide vaccines against diseases such as meningococcal meningitis is limited because they donot stimulate a T cell response, which is required to activatelong term immunological memory. Also, polysaccharide vaccinesinduce poor responses in infants. Up to 40% of cases of meningitisin the United Kingdom are caused by Neisseria meningitidis typec; N meningitidis is particularly virulent in young babies andteenagers. Because the polysaccharide vaccine against meningitisA and C does not induce the production of immunological memorycells and is relatively ineffective in children aged under 2,it does not protect these vulnerable groups.

Conjugate vaccines are being developed to overcome this lack of long term memory. Conjugation involves attaching the requiredpolysaccharide component of the bacteria against which the immuneresponse is to be directed to a protein. This conjugated complexthen behaves more like a protein and is presented and processedin a T cell dependent manner. Activated T cells then secrete cytokinesthat elicit a long term memory response, which includes the productionof antibodies and cell mediated immunity. This response occursat all ages.

Conjugated vaccines against meningitis A and C are likely to undergo clinical trials within two years. Developing a vaccineagainst meningitis B is harder because the risk of a cross reactionbetween certain HLA antigens and the B capsule polysaccharide,which could result in an inappropriate autoimmune response.

Conjugate vaccines are also being developed against pneumococcal disease: 11 of the commonest childhood pneumococcal serotypeshave been conjugated together with a protein in one vaccine, whichshould offer long term immunity against otitis media, communityacquired pneumonia, septicaemia, and meningitis. This vaccineis being assessed in clinical trials; results are expected in1999 $---$ Abi Berger, Science editor, BMJ.