Regulation of the antibody repertoire through control of HCDR3 diversity

Abstract

In man, as in mouse, diversification of the antibody repertoire appears to follow a strict developmental program whereby antigen specificities are serially acquired during ontogeny. When compared to the adult repertoire, the fetal antibody repertoire is highly enriched for polyreactive specificities of low affinity. Although the mechanisms governing the development of this fetal repertoire differ between human and mouse, the composition and structure of the fetal antibodies produced by both species are quite homologous. Specifically, both species use similar V gene segments and restrict the sequence and structure of the third complementarity determining region (HCDR3) of the antibody heavy chain. The precise role that this restriction of the HCDR3 might play in the development of immunocompetence in the human remains to be elucidated.

Figure 1

A comparison of the average hydropathicity of HCDR3 intervals in the mouse: (A) fetal non-productive VDJ joins from liver; (B) fetal productive VDJ joins from liver; (C) adult productive VDJ joins from spleen; (D) adult productive VDJ joins from λ₅/λ₆ ‘knock out’ mice. For (C) and (D), the identity of the reading frame by deletion is noted (RF1, 2 or 3). (Sequence analysis was performed on sequences from Feeney and Löffert et al).

Figure 2

Depicted is D_H and J_H and the distribution of H chain CDR3 lengths as found in 13 Cμ⁺ VDJ transcripts from five fetal liver samples ranging in age from 54 to 76 days gestation, 32 Cμ⁺ VDJ transcripts from 104 and 130 day fetal liver, and 14 cord mononuclear cell VDJ Cμ⁺ transcripts from one sample of cord blood mononuclear cells. On the left are the percentage of transcripts that utilize members of the designated D_H families and on the right are the percentage of transcripts that utilize the designated J_H gene segment. In the middle is the distribution of the lengths of the CDR3 intervals of these transcripts (residues 93–102) divided into three residue intervals (e.g. <9, 10–12, 13–15, 16–18, 19–21, 22–24, and >25 codons).

Figure 3

D_H reading frame utilization as a function of B cell development in man. D_H reading frame was assigned as Term (termination codon), Non-neutral (average polarity < −0.5 or > 0.5, see proposal for details), and Neutral (average polarity <0.5 and > −0.5). Shown is the D_H reading frame for 47 DJ transcripts (D_HQ52 and D_HDXP, 22 VDJ transcripts from pre-B, and 39 VDJ transcripts from B cells where D_H identity could be established. Polarities were calculated using the Kyte-Doolittle hydropathicity index.

Figure 4

Polarity per amino acid of human H chain CDR3s. DJ and VDJ transcripts from pre-B and B cells from second trimester fetal liver have been analyzed. The hydropathicity of each amino within the CDR3 domain of the transcript was determined and the average hydropathicity per amino acid calculated. Shown in panel A is the average hydropathicity of the CDR3 domain of 47 DJ transcripts from pre-B cells. Panel B illustrates the average polarity of the CDR3 domain of 22 VDJ transcripts from the same pre-B cell fraction. Shown in panel C is the average polarity of the CDR3 domain of 64 VDJ transcripts from the B cell fraction.