Bone Marrow Transplantation, Fetal B-Cell Repertoire Development, and the Mechanism of Immune Reconstitution

To the Editor:

Studies of bone marrow (or stem cell) transplantation (BMT) provide important insights in immunological and genetic mechanisms that form the human immune system. Guillaume et al1 published an excellent review of posttransplant B- and T-cell repertoires in the context of therapeutic strategies that could enhance the outcome of BMT. However, they suggested that immune reconstitution after BMT follows a fetal program of development, and I think this warrants further discussion.

The early post-BMT B-cell repertoire is usually characterized as fetal because it appears to be dominated by V_H elements that are frequently detected in fetal liver (most particularly V_H6). Formation of the human fetal V_H repertoire has long been thought to be guided by the location of the V_H elements, with J_H-proximal V_H segments (such as V_H6) rearranging most frequently. However, current evidence contradicts this mechanism.2-4 For instance, analysis of V_H6 expression in fetal tissue with a monoclonal antibody failed to support increased V_H6 levels as determined by random sequencing and Northern blot analysis.2Overexpression of the V_H6 element can therefore no longer be regarded as a characteristic of the fetal repertoire. In addition, because the B-cell repertoire is dominated by (oligo)clonal expansions early after BMT,5-7 measurement of V_H family expression levels may not be the most suitable marker to distinguish fetal- or adult-type immune reconstitution. It is possible that patterns of V_H expression determined by Northern blotting, random sequencing, or V_H family-specific polymerase chain reaction are skewed by dominant clones that express particular V_H families.

What then defines a fetal repertoire? Fetal antigen receptors characteristically contain antigen-binding pockets that are encoded by relatively short third complementarity-determining regions (CDR3).3,4,8 The importance of this characteristic is reflected by the fact that it is conserved through evolution—it has been described in species as diverse as frogs, rabbits, mice, and humans and holds true for both B- and T-cell receptors. Prime determinants of CDR3 size are the usage pattern of diversity (D_H) elements and the length and frequency of N-regions—stretches of DNA that are added by the enzyme terminal deoxynucleotidyl-transferase during formation of the antigen receptor gene. As compared with the situation in the adult, N-regions are expressed at lower frequencies during fetal development.3,4,8 For instance, an estimated 20% of CDR3 regions in 12- to 14-week-old fetal livers lack N-regions altogether. In addition, up to 50% of fetal B-cell receptor CDR3 segments use the DQ52 D_H element (which is relatively small). By contrast to this fetal pattern of CDR3 diversity, CDR3 regions in adult peripheral blood are longer and more diverse; they rarely express DQ52, and they contain extensive addition of N-regions in all instances.8Given the clear difference between fetal-type and adult-type antigen receptors, the pattern of CDR3 diversity is a more reliable marker for distinction between fetal and adult repertoires.

To date, studies of the post-BMT repertoire5-7 demonstrated that CDR3 regions of reconstituting B cells exhibit none of the characteristics that define a fetal repertoire: they rarely encode DQ52, they exhibit adult patterns of N-region addition, and their general size is indistiguishible from that in adult peripheral blood. This pattern of diversity is identical to that in adult bone marrow pre-B cells (which also produce adult-type CDR3 regions).7,9 10 In other words, there is no reason to expect a recapitulation of fetal development after BMT, because the graft, consisting of adult lymphoid progenitors, is placed in the adult environment of the recipient.

In conclusion, immune reconstitution after BMT follows many established ontogenetic patterns relating to the appearance of particular membrane markers, Ig subclasses, and onset of antigen receptor rearrangements. The sequence of events that occur during successful BMT can be regarded as a blueprint for immune reconstitution in other clinical settings as well. However, in the description and interpretation of these events, it is important to realize that immune reconstitution does not appear to recapitulate human fetal ontogeny.