Molecular characterization of variable heavy and light chain regions of five HIV type 1-specific human monoclonal antibodies

Preferential use of the VH5-51 gene segment by the human immune response to code for antibodies against the V3 domain of HIV-1

Human anti-V3 monoclonal antibodies (mAbs) generated from HIV-1 infected individuals display diversity in the range of their cross-neutralization that may be related to their immunogenetic background. The study of the immunoglobulin (Ig) variable region gene usage of heavy chains have shown a preferential usage of the VH5-51 gene segment which was detected in 35% of 51 human anti-V3 mAbs. In contrast, human mAbs against other envelope regions of HIV-1 (anti-Env), including the CD4-binding domain, the CD4-induced epitope, and gp41 preferentially used the VH1-69 gene segment, and none of them used the VH5-51 gene. Furthermore, the usage of the VH4 family by anti-V3 mAbs was restricted to only one gene segment, VH4-59, while the VH3 gene family was used at a significantly lower frequency by all of the analyzed anti-HIV-1 mAbs. Multivariate analysis showed that usage of VH gene segments was significantly different between anti-V3 and anti-Env mAbs, and compared to antibodies from healthy subjects. In addition, the anti-V3 mAbs preferentially used the JH3 and D2-15 gene segments. The preferential usage of selected Ig gene segments and the characteristic pattern of Ig gene usage by anti-V3 mAbs can be related to the conserved structure of the V3 region.

Molecular determinants of the human antibody response to HIV-1: implications for disease control

Various aspects of the immune response to HIV-1 infection remain unclear. While seropositive subjects generally mount a strong humoral response, the antibodies produced are not effective in halting disease progression. Molecular characterization of the antibody repertoire specific for HIV-1 antigens represents an approach to further our understanding of the mechanisms involved in mounting a humoral immunity in this infection. Recently, the content, structure, and organization of the human immunoglobulin-variable gene loci have been elucidated and a number of laboratories have characterized the variable gene elements of human anti-HIV-1 antibodies derived from infected persons by cell fusion or by Epstein-Barr virus transformation. The results show evidence for extensive somatic mutations that lead to preferential amino acid substitutions in the hypervariable regions, an indication of an antigen-driven process. Multiple other molecular events also are engaged in generating antibody diversity, including various types of fusions of variable genes, usage of inverted diversity genes, and addition of extragenomic nucleotides. Most importantly, there is a paucity of antibodies expressing the major V(H)3 gene family, which could result from the capacity of gp120 to act as superantigen for human B cells. This V(H)3+ antibody deficit also has been observed in B cells isolated ex vivo from the patients. Since V(H)3+ antibodies play an essential role in immune defense against infections, the abnormalities observed in HIV-1 infection may predispose to opportunistic infections and further compromise the immune defense mechanisms of the subjects.

Somatic insertions and deletions shape the human antibody repertoire

We have sequenced the heavy and light chain genes from 365 IgG(+) B cells and found that 24 (6.5 %) contain somatically introduced insertions or deletions. These insertions and deletions are clustered at "hot-spots" in the antigen-binding site and frequently result in the creation of new combinations of canonical loop structures or entirely new loops that are not present in the human germline repertoire, but are similar to those seen in other species. Somatic insertion and deletion therefore provides a further mechanism for introducing structural diversity into antibodies in addition to somatic point mutation and receptor editing, which have small (single amino acid changes) and large (chain replacement) impacts on structural diversity, respectively.

Molecular characterization of five neutralizing anti-HIV type 1 antibodies: identification of nonconventional D segments in the human monoclonal antibodies 2G12 and 2F5

We have stabilized a panel of 33 hybridomas producing human monoclonal antibodies (MAbs) against HIV-1 gp160 and p24. Five of these antibodies were able to neutralize different HIV-1 isolates, and two of them (2F5 and 2G12) revealed remarkable potential to neutralize primary virus isolates of different clades in several in vitro tests. To determine whether a structural basis for neutralization could be identified, we analyzed the antibodies at the molecular level. This study reports the primary nucleotide and deduced amino acid sequences of the rearranged heavy and light chain V segments (VH, Vkappa) of the neutralizing MAbs (1B1, 1F7, 2F5, 2G12, and 3D5) and the nonneutralizing anti-gp41 MAb 3D6. Aligning the V segments with the nearest related germline genes illustrated the occurrence of somatic mutations. The neutralizing MAbs show mutational rates comparable to those of antibodies that appear in patients in whom the immune system is under constant antigenic pressure over a long period of time. In contrast, 3D6, which recognizes the immunodominant region on gp41, displays homologies as high as 97 and 98% compared with its VH and Vkappa germline genes. The diversity segments [D(H)] of 1B1, 1F7, 3D5, and 3D6 were assigned to single D(H) segments on the chromosomal D(H) locus. 2F5 presents a D(H) segment 52 nucleotides in length, which could be explained by fusion of two segments on the immunoglobulin heavy chain locus that have not yet been described as rearranged regions. 2G12 D(H) shows best homologies to a D(H) segment between D3-22 and D4-23. This D(H) segment could be the reason for the rare occurrence of antibodies competing with 2G12. Since this nearest related chromosomal region on the D(H) locus does not display recombination signals at the flanking regions, this segment is normally not taken into consideration as a site for immunoglobulin heavy chain rearrangement.

The immunology of AIDS-associated lymphomas

Lymphomas that occur in patients with human immunodeficiency virus (HIV) infection are predominantly of B-cell origin and subsets show evidence for Epstein-Barr virus (EBV) infection or chromosomal translocations in the c-myc locus. The only subset of lymphoma clearly related to the immunodeficiency caused by HIV infection (similar to transplantation-associated lymphomas) is the EBV+ primary central nervous system lymphoma. The systemic AIDS-related lymphomas (ARLs) represent a complex set of disease processes histologically categorized as large cell or small non-cleaved (Burkitt's-like) lymphomas. Molecular analyses of the ARLs have demonstrated polyclonal lymphomas as likely early representatives of monoclonal immunoglobulin (Ig)-expressing B-cell lymphomas. Variable region analysis of lymphoma-associated Ig has shown evidence for extensive somatic mutation with little evidence for appropriate affinity maturation. These observations suggest that abnormal control of B-cell maturation in response to polyclonal antigenic stimulation may play a central role in the pathogenesis of ARL. The recent finding of clonal HIV integrated within macrophages in a subset of early lymphomas also provides evidence for abnormalities outside the B-cell compartment playing roles in this disease. Overall, ARLs generally appear to be outgrowths of antigen-driven B-cells with significant growth control influence provided by abnormal T-cell and antigen-presenting cell processes.

VH gene use by HIV type 1-associated lymphoproliferations

The pathogenesis of polyclonal HIV-associated lymphomas lacking traditional B cell cofactors (i.e., Epstein-Barr virus [EBV] infection, c-myc translocations) is poorly understood. A multistep pathogenesis model has been proposed in which polyclonal lymphomas represent an earlier stage in HIV-associated lymphomagenesis before the emergence of a dominant malignant clone. Chronically present antigens have been proposed as a likely stimulus for polyclonal B cell proliferation; if so, polyclonal lymphoma-associated immunoglobulins (Igs) should have molecular evidence of somatic hypermutation, a process by which antibody affinity maturation in response to chronic antigenic stimulation occurs. Molecular analyses of Ig heavy chain variable (V(H)) gene use by B cells in a polyclonal HIV-associated large cell lymphoma lacking EBV and c-myc rearrangement was undertaken. Eighteen randomly selected clones generated from RT-PCR yielded 15 unique V(H) sequences, all of which were most homologous to only three previously identified germline V(H)1 genes. Two sets of clones (consisting of three and two clones, respectively) had identical V(H) gene sequences, and one pair of clones had identical third complementarity determining regions (CDR3s) but different V(H) gene sequences; eight clones were <95% homologous to their most related germline V(H)1 genes. We compared these results with Ig V(H)1 gene use by B cells present in a reactive hyperplastic lymph node obtained from an HIV-1-infected individual. Fifteen clones randomly selected from RT-PCRs yielded 15 unique V(H)1 sequences, all of which were most homologous to 5 previously identified germline V(H)1 genes; 10 clones were <95% homologous to their most related germline gene. Binomial probability analysis revealed that only 1 of the 15 unique V(H)1 sequences derived from the polyclonal lymphoma (i.e., 7%), as compared with 5 of 15 unique V(H)1 sequences derived from the reactive lymph node (i.e., 33%), had a low probability of occurrence by random chance (p < 0.05). These data provide molecular evidence of polyclonality in an HIV-associated polyclonal lymphoma, demonstrate a qualitative difference in somatic hypermutations of Ig V(H) genes associated with malignant versus reactive B cell lymphoproliferations, and support an antigen-mediated multistep pathogenesis model of HIV-1-associated lymphomagenesis.

Nonrandom features of the human immunoglobulin variable region gene repertoire expressed in response to HIV-1

Characterization of the immune response toward HIV is important for understanding the basic mechanisms of the disease and may give essential information for development of an anti-HIV vaccine. Paradoxically, although HIV infection is associated with a strong antibody response to structural and nonstructural HIV proteins, this immune response does not seem to halt disease progression. Both quantitative and qualitative B-cell abnormalities are associated with disease progression. The immunological abnormalities in HIV-1 infection include abnormal cytokine production and expansion of HIV-1-specific B-cell precursors that may reach 40%. There is also evidence that gp120 exerts a B-cell superantigen-like activity on human B-cells through binding to gene products of the third heavy-chain variable region family (VH3). This property of gp120 may induce abnormal mechanisms of selection of the antibody repertoire. It may also account for the apparent paucity of anti-gp120 antibodies expressing VH3 genes and for the polyclonal activation seen in the early stages of HIV infection. This expansion would reflect specific stimulation of VH3 B-cells, but not all B-cells. It would then be followed by a significant deletion of this B-cell subset. Finally, autoimmune phenomena have been described in HIV infection, and several hypotheses have been put forward to account for such associations. On the basis of the superantigen concept discussed above, one may suggest that gp120 may trigger B-cell subsets bearing receptors with specificities for self-components. This would explain the multiplicity of autoantibody specificities seen in this disease.

Characteristics of human antibody repertoires following active immune responses in vivo

Possibilities to develop human monoclonal antibody specificities have recently been much facilitated by improvements of human hybridoma technology but even more so by the emerging phage-display technique. However, until recently very little has been known about the characteristics at the molecular level of the induced, T cell-dependent human antibody response, frequently targeted by these techniques. Rather, the major part of available sequence information has been related to tumor-derived or autoreactive antibodies. We have now investigated high affinity, monospecific, human antibody repertoires as developed by hybridoma technology. The VH region gene usage among such in vivo-induced repertoires is in only some respects similar to that found in the total B cell population. A limited number of heavy-chain variable segment loci account for the majority of all induced antibodies. A particular VH gene locus (4-34) frequently employed by peripheral B cells and associated with autoreactive antibodies was rarely used by the induced repertoire. Furthermore, in particular antigen systems, V region usage differs from the total available repertoire, and heavy-chain CDR3 is generally longer among antibodies induced against foreign protein antigens than in the average B cell population. Light-chain gene usage is often restricted to just a few dominant genes frequently found among B cells in general. In comparison, variable regions derived by phage-display technology in some antigen systems display even longer heavy-chain CDR3 than hybridoma-derived antibodies. This technique also appears to select a different set of germline genes preferentially (both with respect to VH and JH) as compared to hybridoma technology. In summary, the T cell-dependent antibody response against foreign antigens appears to differ from the average circulating B cell in several ways, and thus does not seem to represent a random selection of the available repertoire.

B-cell superantigens: implications for selection of the human antibody repertoire

For several decades, B-cell interactions with antigens were thought to occur only through a clonal activation mechanism, in which the hypervariable regions of the immunoglobulin receptor are exclusively involved in ligand binding. However, an additional mode of interaction can occur, whereby molecules, termed B-cell superantigens, can bind human B cells bearing immunoglobulin receptors of a given variable (V)-gene family. This mechanism requires contributions from regions outside the conventional hypervariable loops and results in a B-cell response of increased magnitude. Here, Moncef Zouali reviews recent in vitro and in vivo observations on human B-cell superantigens in the context of the current consensus of B-cell development, and discusses the implications of these novel concepts with respect to pathogenesis.

Molecular characterization of human monoclonal antibodies specific for several HIV proteins: analysis of the VH3 family expression

We have analyzed the heavy-chain variable (VH) region genes expressed by a panel of human monoclonal antibodies derived from an immunized volunteer, an AIDS patient and seropositive asymptomatic donors, and specific for HIV-1 env, pol and gag gene products. The third complementarity-determining regions show a high complexity with unconventional gene recombination events. Most of the VH genes utilized are also frequently encountered in other immune responses. Their sequences are, in general, typical of an antigen-driven immune response. Molecular mechanisms that generate high-affinity antibodies are then effective during HIV infection. Remarkably, VH3 family, which dominates the human antibody repertoire, is barely encountered among anti-HIV antibodies.