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Human immune responses to polysaccharide antigens: an analysis of bacterial polysaccharide vaccines in infants
Insights
Infant immune responses to polysaccharide vaccines are poor due to immature B lymphocytes and regulatory T cells. Understanding these mechanisms is key to developing effective vaccines for bacterial infections in young children.
Area of Science:
- Immunology
- Vaccinology
- Pediatrics
Background:
- Purified polysaccharide bacterial vaccines are available but poorly immunogenic in infants under two years old.
- Infants are at the highest risk for severe infections caused by encapsulated bacteria.
- Existing vaccines have not fully met the promise of preventing childhood bacterial sepsis and meningitis.
Purpose of the Study:
- To characterize the human immune response to polysaccharide antigens in detail.
- To understand the reasons for the poor immunogenicity of polysaccharide vaccines in infants.
- To identify cellular requirements and immunoregulation for effective infant immunization.
Main Methods:
- Review of mechanisms of human immunity to polysaccharide encapsulated bacteria.
- Characterization of the human immune response to polysaccharide antigens, using pneumococcal polysaccharide type 3 as a model.
- Analysis of B lymphocyte subpopulations and T regulatory influences.
Main Results:
- Human antibody responses to polysaccharides resemble T cell-independent type 2 responses seen in mice.
- Infants' deficient polysaccharide response is linked to immature B lymphocyte subpopulations.
- Imbalanced T regulatory influences also contribute to the impaired immune response in infants.
Conclusions:
- The complexity of human antibody responses to polysaccharide antigens has hindered vaccine efficacy in infants.
- Further research into cellular requirements and immunoregulation is crucial.
- A clearer direction exists for developing effective infant immunization strategies against bacterial infections.
Abstract:
Mechanisms of human immunity to polysaccharide encapsulated bacteria and the development and testing of the currently available purified polysaccharide bacterial vaccines are reviewed. These vaccines appear to be poorly immunogenic in infants under the age of two years--those at greatest risk for infection. In an effort to understand the poor responses of infants, the human immune response to polysaccharide antigens was characterized in more detail. Using pneumococcal polysaccharide type 3 as an example, it appears that human polysaccharide antibody responses are analogous to the type 2 T cell independent responses defined in the murine system. These studies suggest that the deficient polysaccharide response of human infants is due to a deficiency in maturation of distinct B lymphocyte subpopulations, as well as imbalanced T regulatory influences. The development of vaccines containing the purified capsular polysaccharides of S. pneumoniae, H.influenzae, and N. meningitidis during the past decade offered promise for the prevention of the major causes of bacterial sepsis and meningitis during childhood. The fulfillment of that promise was thwarted by the unrecognized complexities of human antibody responses to polysaccharide antigens. Continued vigorous research in this area has led us to a better understanding of the cellular requirements and immunoregulation of human polysaccharide antibody responses and has given us a clear direction for the pursuit of an effective means for immunization of infants.