2003; baxter - Supplements - Haematologica

IV International Workshop on Immune Tolerance in Hemophilia 21Figure 2. Variable heavy chain gene segments of the VH3 family present in peripheral IgG-positive B cells. On the x-axis the 22gene segments belonging to the VH3 family are listed. On the y-axis the percentage of IgG-positive B cells expressing an IgGcontaining one of these gene segments is depicted (data drawn from ref. #21). Anti-A2 and anti-A3-C1 antibodies are derivedfrom gene segments 3-21, 3-23 and 3.30 which are commonly expressed in peripheral IgG-positive B-cells.All isolated antibodies were derived fromgermline gene segments of the VH1 and VH3family. Two of the anti-A3-C1 antibodies werederived from germline gene segment DP-49 (3-30), two of DP-77 (3-21), one of DP-14 (1-18)and one of DP-15 (1-8). At first sight there is littleto be learned from the origin of variable heavychain of the anti-A2 and anti-A3 antibodies.Germline gene segments used are derived fromboth the VH1 and VH3 family which does notcome as a surprise since the VH segments fromthese families are used by 70% of peripheral IgG +B-cells (see Figure 1). Closer inspection of theoccurrence of the different VH gene segments inthe normal repertoire yields some remarkablefeatures (Figure 2). About 25% of the humanIgG + repertoire is derived from VH gene segmentsDP-47 (3-23), DP-49 (3-30) and DP-77 (3-21).These findings show that anti-A2 and anti-A3antibodies use VH gene segments that are preferentiallyexpressed in IgG molecules in the normalrepertoire. The observed preference suggeststhat epitopes present in the A2 and A3-C1domains of factor VIII are accessible forimmunoglobulins that have incorporated VHsegments DP-47 (3-23), DP-49 (3-30) and DP-77 (3-21). The high concentration of IgG + B cellsin the periphery that are derived from these VHgene segments may be due to some inherent flexibilityof IgG molecules with these VH gene segmentsto bind to antigenic sites on a variety ofproteins. Alternatively, IgG molecules containingthese VH gene segments may more efficientlycope with selective processes that occur duringmaturation of B-cells. A relatively large numberof immature B-cells containing surface IgGderived from these VH gene segments is thenavailable for incoming antigen. Independently ofthe underlying mechanism, the presence of DP-47 (3-23), DP-49 (3-30) and DP-77 (3-21) genesegments in anti-A2 and anti-A3-C1 antibodiessuggests that antibodies with this specificity arefrequently observed in plasma of inhibitorpatients. Indeed a large study concluded thatanti-A2 or anti-A3 antibodies are present in virtuallyall patients with factor VIII inhibitors. 28The above analysis provides an attractively simpleexplanation for the frequent occurrence ofanti-A2 and anti-A3 antibodies in inhibitorpatients. However, some caution is warranted.So far only a limited number of patients has beenincluded in our analysis. Also, our studies suggestthat the epitopes in the A2 and A3-C1 domainsare more complex then previously anticipated.24,25 More detailed studies on epitope specificityand VH gene usage are required in order todetermine unequivocally whether a particular VHgene segment is preferentially incorporated intoan IgG molecule binding to an antigenic site inthe A2 or A3-C1 domain of factor VIII.Characteristics of anti-C2 antibodiesobtained by phage displayThe plasma of most inhibitor patients containsantibodies that react with the C2 domain of factorVIII. 28 At present we have isolated 16 differentscFv reactive with this domain from therepertoire of patients with mild and acquiredhaematologica vol. 88(supplement n. 12):september 2003