B CELLS AND ANTIBODY RESPONSE

M. Jeyakanthan¹, X. Zhou¹, K. Tao¹, R.V. Rajotte², M. Mengel³, L.J. West^4
1Pediatrics, University of Alberta, Edmonton/AB/CANADA, ²Surgery, University of Alberta, Edmonton/AB/CANADA, ³Department Of Laboratory Medicine And Pathology, University of Alberta, Edmonton/AB/CANADA, ⁴Department Of Pediatrics, Cardiac Transplant Research, University of Alberta, Edmonton/AB/CANADA

Body: Introduction: ABO-incompatible infant heart transplant results in development of B-cell tolerance to donor A/B antigens, suggesting the inherent susceptibility of immature immune system to tolerance induction. Blood group antigen is expressed only on vascular endothelium in human heart. We sought to determine in a relevant pig model whether the vascular endothelium site of blood group antigen expression is necessary for the induction of B-cell tolerance during immunologic immaturity.

Methods: Developmental profile of isoagglutinin production in piglets was established to identify a window of immunological immaturity. Serial blood samples were tested for anti-A/B antibody of IgM, IgG and IgA isotypes by ABO-ELISA in type O outbred piglets from birth to 4 months. Histo-blood group antigen expression was assessed by immunohistochemistry of tissue sections from A and O type pigs using monoclonal antibodies to A and H. Two groups of O piglets received single kidney transplants from either A donor (AO-incompatible group: n=4) or O donor (AO-compatible group: n=4). All recipients received daily oral cyclosporine guided by the blood level. Graft rejection and persistence of antigen expression were monitored by multiple kidney biopsies; post-operative development of anti-A/B antibody was monitored by serial blood sampling over 3 months. Renal allograft biopsies were interpreted according to Banff classification.

Results:Anti-A/B IgM was deficient between 3-4 weeks after birth, whilst IgG and IgA were deficient until 7 weeks. Kidney sections from type A and O pigs showed strong antigen expression in renal tubules but no expression in capillary and vascular endothelium. First biopsy at 2-3 weeks revealed either borderline or no rejection of allografts in both groups. Two pigs in AO-incompatible group had their allografts completely resorbed at the end of the experiment despite a normal biopsy at 2 weeks. One pig from AO-compatible group had grade III (i2 t3 v3) rejection at 90 days post op. Immunohistochemistry of tissue sections confirmed A or H antigen expression on all biopsy samples taken peri- and post-operatively. Anti-A antibody development was observed in both AO-incompatible and AO-compatible groups.

Conclusions: Natural antibody development in piglets is analogous to human infants. We identified a window of isoagglutinin deficiency in neonatal pigs during which B cell tolerance to A/B antigens may be induced. We successfully developed an AO-incompatible kidney transplant model in 3 week old piglets and were able to maintain kidney allograft with cyclosporine monotherpy. Despite persistent antigen expression, AO-incompatible kidney graft recipients developed blood group antibody normally, in contrast to the pattern observed in ABO-incompatible infant heart transplants. Since we found that in the kidney, blood group antigens are expressed in the tubules, and lacking on vascular endothelium, these results suggest that blood group antigen expression on vascular endothelium may be required for B cell tolerance induction to carbohydrate antigens following ABO-incompatible transplant during immunological immaturity.

Disclosure: All authors have declared no conflicts of interest.