Pneumococcal Capsular Polysaccharide Immunity in the Elderly

Abstract

Immunity to pneumococcal infections is impaired in older people, and current vaccines are poorly protective against pneumococcal disease in this population. Naturally acquired immunity to pneumococcal capsular polysaccharides develops during childhood and is robust in young adults but deteriorates with advanced age. In particular, antibody levels and function are reduced in older people. Pneumococcal vaccines are recommended for people >65 years old. However, the benefits of polysaccharide and protein-conjugated vaccines in this population are small, because of both serotype replacement and incomplete protection against vaccine serotype pneumococcal disease. In this review, we overview the immune mechanisms by which naturally acquired and vaccine-induced pneumococcal capsular polysaccharide immunity declines with age, including altered colonization dynamics, reduced opsonic activity of antibodies (particularly IgM), and impaired mucosal immunity.

Keywords: Streptococcus pneumoniae; aging; colonization; immunization; immunoglobulins; mucosal immunity; pneumococcus; pneumonia.

FIG 1

Anticapsular antibodies can be acquired naturally (following pneumococcal exposure, e.g., colonization, or through pneumococcal disease) or via vaccination. They facilitate pneumococcal killing via opsonization. In addition, they can prevent the development of colonization in the future—this has been shown to be mediated via agglutination in the case of antibodies induced by protein-conjugated pneumococcal vaccines.

FIG 2

Schematic of pneumococcal disease rates, pneumococcal colonization rates, and pneumococcal antibody activity in different age groups. Pneumococcal colonization and disease rates are high in young children. Naturally acquired pneumococcal anti-CPS antibody levels rise with recurrent exposure. Young adults have high levels of naturally acquired antibodies, occasional episodes of colonization, and low rates of disease. In the elderly, antibody levels are low and functional activity is even lower, colonization is infrequent, and rates of pneumococcal disease increase.