Development of B-cell memory in early childhood and the impact on antigen-specific tolerance after heart transplantation.

BACKGROUND: Young children show better survival after heart transplant compared with older individuals and can receive heart transplants safely from ABO-incompatible donors. Children develop immunologic tolerance to donor ABH antigens reflected in persistent absence of specific antibodies. We hypothesized that immature T-independent B-cell response and lack of B-cell memory play a crucial role in tolerance of ABH antigens after ABOi transplants. METHODS: We determined phenotypes of splenic lymphocytes from adults and children and peripheral blood from ABO-incompatible or ABO-compatible heart transplant recipients and control subjects by flow cytometry. In vitro immune response to T-independent stimulation, erythrocytes, and ABH antigens was assessed using proliferation assays. RESULTS: A predominant role for CD27(+) B cells in T-independent activation was demonstrated; these cells were significantly less frequent in infants than older subjects. Only IgM(+)CD27(+) B cells proliferated in response to non-self erythrocytes. In blood, IgM(+) and switched IgM(-) memory B cells were rare in infants, increasing to near-adult levels in children 5 years old. IgM(+)CD27(+) B cells were significantly fewer in ABO-incompatible transplant recipients than in ABO-compatible recipients. CONCLUSIONS: CD27(+) cells play a key role in T-independent B-cell activation. Response to ABH antigens is mediated by IgM(+)CD27(+) B cells, and donor ABO-specific tolerance after ABO-incompatible transplantation in children is facilitated by low prevalence of these cells. The pattern of B-cell memory development is altered after ABO-incompatible transplant. Memory B cells may be quantified to assess eligibility for ABO-incompatible transplant and represent a potential therapeutic target to extend the benefits of the immature immune system to older age groups.

C3d plasma levels and CD21 expressing B-cells in children after ABO-incompatible heart transplantation: Alterations associated with blood group tolerance.

BACKGROUND: Most children transplanted with ABO-incompatible (ABOi) hearts develop selective tolerance to donor A/B antigens, whereas anti-A/B antibodies typically re-accumulate in adults after ABOi kidney transplantation. Deficiency of essential factors linking innate and adaptive immunity in early childhood may promote development of tolerance, specifically interactions between complement split product C3d and its ligand CD21 on B cells, considering their role in augmenting "T-independent" B-cell activation. METHODS: Blood and clinical data were analyzed from children after ABOi or ABO-compatible (ABOc) heart transplantation (HTx). Plasma C3d levels were quantified by enzyme-linked immunoassay. Peripheral blood mononuclear cells (PBMC) were phenotyped by flow cytometry; expression of B-cell co-receptor components CD21 and CD81 was quantified. RESULTS: Fifty-five samples from pediatric HTx recipients (median age at transplant: 4.2 [range 0.03 to 20.4] months; age at sample collection: 14.6 [0.04 to 51.3] months; 53% ABOi) and 21 controls were studied. CD21-expressing B cells increased in trend with age (p = 0.079); longitudinal measures in individual patients showed a strong correlation with age. CD21 expression intensity in B-cells was not age-dependent. Plasma C3d levels did not correlate with age. Comparing ABOc vs ABOi HTx, CD21-expressing cell proportions were similar; however, serum C3d levels were significantly lower after ABOi HTx (p < 0.05). CONCLUSIONS: In children, including HTx patients, CD21-expressing B-cells show a trend to increase with age, corresponding with improved responsiveness to polysaccharide antigens. This does not differ in patients with ABOi grafts developing tolerance to donor ABO antigens. C3d levels are not age-dependent, but reduced C3d levels after ABOi HTx suggest altered complement metabolism contributing to ABO tolerance.