Defective anticarbohydrate antibody responses to naturally occurring bacteria following bone marrow transplantation

Humoral Immune Reconstitution Kinetics after Allogeneic Hematopoietic Stem Cell Transplantation in Children: A Maturation Block of IgM Memory B Cells May Lead to Impaired Antibody Immune Reconstitution

Although T cell immune reconstitution after allogeneic hematopoietic stem cell transplantation (allo-HSCT) has been well studied, long-term B cell immune reconstitution remains less characterized. We evaluated humoral immune reconstitution among 71 pediatric allo-HSCT recipients. Although tetanus toxoid antibody levels were normal at 1 year after allo-HSCT, antipolysaccharide carbohydrate antibodies remained persistently low for up to 5 years. While naive B cell counts normalized by 6 months, IgM memory B cell deficiency persisted for up to 2 years (P = .01); switched memory B cell deficiency normalized by 1 year after allo-HSCT. CD4⁺ T cell immune reconstitution correlated with that of switched memory B cells as early as 6 months after allo-HSCT (r = .55, P = .002) but did not correlate with IgM memory B cells at any time point after allo-HSCT. Taken together, this suggests that allo-HSCT recipients have impaired antibody immune reconstitution, mainly due to IgM memory B cell maturation block, compared with more prompt T cell-dependent switched memory cell immune reconstitution. We further explored other factors that might affect humoral immune reconstitution. The use of total body irradiation was associated with lower naive B cells counts at 6 months after HSCT (P = .04) and lower IgM (P = .008) and switched (P = .003) memory B cells up to 2 years. Allo-HSCT recipients with extensive chronic graft-versus-host disease had lower IgM memory B cell counts (P = .03) up to 2 years after allo-HSCT. The use of cord blood was associated with better naive (P = .01), IgM (P = .0005), and switched memory (P = .006) B cells immune reconstitution. These findings may inform future prophylaxis and treatment strategies regarding risk of overwhelming infection, graft-versus-host disease, and post-allogeneic HSCT revaccination.

Antigen-specific immunity following hematopoietic stem cell transplantation

Most studies evaluating immune reconstitution following hematopoietic stem cell transplantation (HSCT) have focused on immunophenotypic analysis and the capacity of the immune system to respond to mitogenic stimulation. However, protection against infectious pathogens and potentially anti-tumor responses correlate with the presence of antigen-specific immunity, not the immunophenotypic presence of T lymphocytes. Antigen-specific T lymphocytes present after HSCT can be derived from donor antigen-specific T lymphocytes present in the transplantation inoculum if it is not T cell depleted. Furthermore, the naive T lymphocytes contained in the HSCT inoculum have the potential to develop into antigen-specific T lymphocytes. If the transplantation inoculum is T cell depleted, then all antigen-specific T lymphocytes will have to be derived from the newly engrafted hematopoietic stem cells following their differentiation through the recipient thymus. Thus, defects in thymopoiesis will result in delays or the absence of naive T lymphocytes and ultimately defects in antigen-specific immunity.

Recommendations for immunizations in stem cell transplantation

Investigations over the past decade have documented that there is a decline in immunity to vaccine preventable diseases in many SCT recipients. The majority of immunization studies conducted in SCT recipients to date support the use of multi-dose regimens for most protein and polysaccharide-conjugate vaccine antigens. The consensus immunization schedule recommended by ACIP/IDSA/ASBMT provides guidance for centers to utilize available vaccines in their SCT populations. With the exception of pneumococcal disease, a schedule beginning at 12 months after SCT is reasonable given the low incidence of disease in HSCT recipients for most of the recommended vaccines and improved immune reconstitution in most recipients by one year post transplant. SCT recipients respond poorly to unconjugated pneumococcal polysaccharide vaccine and the development of polysaccharide-protein conjugate vaccines against S. pneumoniae holds promise to impact potentially on clinical disease in this population. In addition, the strategy of donor immunization may also be effective in eliciting early protective immune responses to vaccine antigens. Future challenges will be the development of safe and effective vaccines against the viral pathogens responsible for considerable morbidity and mortality after SCT.

Antigen-specific immune function after hematopoietic stem cell transplantation

Hematopoietic stem cell recipients are characterized by an immunodeficiency of varying severity and duration. The present review focuses on the antigen-specific function of recipients with the hypothesis that the acquisition of antigen-specific function is predictive of the recipient's capacity to resist lethal infection with environmental pathogens.