IgG+, CD5+ human chronic lymphocytic leukemia B cells. Production of IgG antibodies that exhibit diminished autoreactivity and IgG subclass skewing

Chronic Lymphocytic Leukemia B-Cell Normal Cellular Counterpart: Clues From a Functional Perspective

Chronic lymphocytic leukemia (CLL) is characterized by the clonal expansion of small mature-looking CD19+ CD23+ CD5+ B-cells that accumulate in the blood, bone marrow, and lymphoid organs. To date, no consensus has been reached concerning the normal cellular counterpart of CLL B-cells and several B-cell types have been proposed. CLL B-cells have remarkable phenotypic and gene expression profile homogeneity. In recent years, the molecular and cellular biology of CLL has been enriched by seminal insights that are leading to a better understanding of the natural history of the disease. Immunophenotypic and molecular approaches (including immunoglobulin heavy-chain variable gene mutational status, transcriptional and epigenetic profiling) comparing the normal B-cell subset and CLL B-cells provide some new insights into the normal cellular counterpart. Functional characteristics (including activation requirements and propensity for plasma cell differentiation) of CLL B-cells have now been investigated for 50 years. B-cell subsets differ substantially in terms of their functional features. Analysis of shared functional characteristics may reveal similarities between normal B-cell subsets and CLL B-cells, allowing speculative assignment of a normal cellular counterpart for CLL B-cells. In this review, we summarize current data regarding peripheral B-cell differentiation and human B-cell subsets and suggest possibilities for a normal cellular counterpart based on the functional characteristics of CLL B-cells. However, a definitive normal cellular counterpart cannot be attributed on the basis of the available data. We discuss the functional characteristics required for a cell to be logically considered to be the normal counterpart of CLL B-cells.

From pathogenesis to treatment of chronic lymphocytic leukaemia

Chronic lymphocytic leukaemia (CLL) has several unique features that distinguish it from other cancers. Most CLL tumour cells are inert and arrested in G0/G1 of the cell cycle and there is only a small proliferative compartment; however, the progressive accumulation of malignant cells will ultimately lead to symptomatic disease. Pathogenic mechanisms have been elucidated that involve multiple external (for example, microenvironmental stimuli and antigenic drive) and internal (genetic and epigenetic) events that are crucial in the transformation, progression and evolution of CLL. Our growing understanding of CLL biology is allowing the translation of targets and biological classifiers into clinical practice.

Structural and Functional Features of the B-Cell Receptor in IgG-Positive Chronic Lymphocytic Leukemia

PURPOSE

To determine the origin and relationship of the rare IgG+ variant of chronic lymphocytic leukemia (CLL) to the two common IgM+IgD+ subsets that are distinguished by expression of unmutated or mutated V(H) genes, with the former having a worse prognosis.

EXPERIMENTAL DESIGN

IgG+ CLL cells were characterized using phenotypic, functional, and immunogenetic analyses.

RESULTS

IgG+ CLL was phenotypically similar to mutated IgM+IgD+ CLL (M-CLL) and variably expressed CD38 (4 of 14). ZAP-70, a tyrosine kinase preferentially expressed in unmutated CLL, was found in only 2 of 14 cases. The ability to signal via surface IgM (sIgM) varies between the main subsets of CLL and is associated with expression of ZAP-70. In IgG(+) CLL, 9 of 14 responded to engagement of sIgG with no apparent requirement for expression of CD38 or ZAP-70. However, signal capacity correlated with intensity of sIgG expression. Most switched immunoglobulin variable region genes were somatically mutated without intraclonal variation, and no case expressed activation-induced cytidine deaminase. Derivation from a postgerminal center B cell is, therefore, likely, and a relationship with M-CLL is suggested. This is supported by a shared biased usage of the V4-34 gene. Similar bias in normal B cells developed with age, providing an expanded population for transforming events. However, conserved sequences detected in the CDR3 of V4-34-encoded gamma chains were not found M-CLL, indicating no direct path of isotype switch from M-CLL.

CONCLUSION

IgG+ CLL is likely to arise from an age-related expanded pool of B cells, on a path parallel to M-CLL, and perhaps with a similar clinical course.

Clinical and laboratory parameters that define clinically relevant B-CLL subgroups

B cell-type chronic lymphocytic leukemia (B-CLL) is a heterogeneous disease. This is reflected by the very wide-ranging clinical courses that B-CLL patients experience and by the marked variation in laboratory findings between patients. In this chapter, we will review the various clinical and laboratory parameters that divide B-CLL patients into "subgroups," and correlate the parameters that define them. When feasible, we will also link clinical features to the cellular and genetic characteristics recently defined for these leukemic cells. The discussion is limited to parameters that define phenotypes or subgroups that may relate to disease activity and clinical outcome.

Recent advances in the molecular biology and immunobiology of chronic lymphocytic leukemia

B-cell chronic lymphocytic leukemia (B-CLL) has long been viewed as a relatively homogeneous disease caused by the accumulation of monoclonal immature, immunoincompetent B cells with faulty apoptotic capacities. However, recent evidence, reviewed here, demonstrates that at least two different B-CLL subgroups exist with different clinical courses and outcomes. The malignant cells from both B-CLL subgroups are antigen-experienced cells that have a normal apoptotic apparatus and turnover continually. The leukemic cells of the two B-CLL subgroups have engaged antigen before transformation, although primarily the cells of patients in the poor outcome subgroup can respond to antigens following transformation. The difference in the ability to respond to antigen as a full-fledged B-CLL probably accounts for the different biological features and clinical outcomes of the patients in these subgroups.

Remarkably similar antigen receptors among a subset of patients with chronic lymphocytic leukemia

Studies of B cell antigen receptors (BCRs) expressed by leukemic lymphocytes from patients with B cell chronic lymphocytic leukemia (B-CLL) suggest that B lymphocytes with some level of BCR structural restriction become transformed. While analyzing rearranged V(H)DJ(H) and V(L)J(L) genes of 25 non-IgM-producing B-CLL cases, we found five IgG(+) cases that display strikingly similar BCRs (use of the same H- and L-chain V gene segments with unique, shared heavy chain third complementarity-determining region [HCDR3] and light chain third complementarity-determining region [LCDR3] motifs). These H- and L-chain characteristics were not identified in other B-CLL cases or in normal B lymphocytes whose sequences are available in the public databases. Three-dimensional modeling studies suggest that these BCRs could bind the same antigenic epitope. The structural features of the B-CLL BCRs resemble those of mAb's reactive with carbohydrate determinants of bacterial capsules or viral coats and with certain autoantigens. These findings suggest that the B lymphocytes that gave rise to these IgG(+) B-CLL cells were selected for this unique BCR structure. This selection could have occurred because the precursors of the B-CLL cells were chosen for their antigen-binding capabilities by antigen(s) of restricted nature and structure, or because the precursors derived from a B cell subpopulation with limited BCR heterogeneity, or both.

Activation-induced cytidine deaminase in chronic lymphocytic leukemia B cells: expression as multiple forms in a dynamic, variably sized fraction of the clone

The degree of somatic mutation of immunoglobulin variable (Ig V) region genes is an important prognostic indicator of clinical course and outcome in B-cell chronic lymphocytic leukemia (B-CLL), although the reason for this association remains unclear. Furthermore, some B-CLL cells continue to acquire Ig V gene mutations after the transforming event. Because activation-induced cytidine deaminase (AID) is an essential component of the canonical somatic hypermutation process in healthy B cells, its expression in B-CLL is potentially relevant to the disease. We detected full-length AID transcripts and 3 splice variants by conventional reverse transcription polymerase chain reaction (RT-PCR) in approximately 40% of the cases examined. More sensitive real-time quantitative PCR detected AID transcripts in virtually all B-CLL samples tested, although the range of transcript levels was very large between different cases and varied within individual cases over time. Limiting dilution assays revealed that AID expression was restricted to a small fraction of the leukemic cells in the blood. However, this small fraction is not unique in its ability to express AID, because in vitro stimulation of B-CLL cells with appropriate stimuli significantly increased the fraction of AID-expressing cells. These data suggest that AID-mediated DNA alterations may occur in a variably sized, minor subset of B-CLL cells at any given time.

B cell chronic lymphocytic leukemia: lessons learned from studies of the B cell antigen receptor

B cell chronic lymphocytic leukemia (B-CLL) is an accumulative disease of slowly proliferating CD5(+) B lymphocytes that develops in the aging population. Whereas some patients with B-CLL have an indolent course and die after many years from unrelated causes, others progress very rapidly and succumb within a few years from this currently incurable leukemia. Over the past decade studies of the structure and function of the B cell antigen receptor (BCR) used by these leukemic cells have helped redefine the nature of this disease. In this review we summarize and reinterpret several aspects of these BCR-related studies and how they might relate to the disease. In particular, we address the ability of antigens to select out and drive B cell clones from the normal state to overt leukemic cells by binding to BCRs that are relatively unique and characteristic of B-CLL cells. The differential capacity of some B-CLL cases to continue to transduce signals through the BCR during the leukemic phase and the consequences for the in vivo biology of the leukemic clone is also considered. Finally, we discuss current and emerging views of the cellular origin of B-CLL cells and the differentiation pathways down which we believe these cells progress.

Reactivity of serum IgG anti-GM1 ganglioside antibodies with the lipopolysaccharide fractions of Campylobacter jejuni isolates from patients with Guillain-Barre syndrome (GBS)

Campylobacter jejuni (Cj) enteritis is the most frequently recognised infection preceding Guillain-Barre syndrome (GBS) and this combination is commonly associated with anti-GM1 ganglioside (anti-GM1) antibodies. We have examined the hypothesis that the anti-GM1 antibodies represent an immune response against the Cj lipopolysaccharide (LPS). We prepared the LPS fraction from 8 isolates of Cj, 3 from GBS patients with acute inflammatory demyelinating polyradiculoneuropathy (AIDP), 3 from patients with Miller Fisher syndrome (MFS) and 2 from enteritis patients without neurological disease. We looked for IgG antibodies against LPS and GM1 in the serum of 10 GBS and 10 MFS patients, including the patients from whom the Cj had been isolated, and 11 normal control subjects. The highest levels of IgG binding to LPS fractions were found in the GBS patient sera and were with one of the LPS fractions extracted from the C. jejuni isolated from a GBS patient, one from a MFS patient and two Cj isolates from enteritis patients without neurological disease. The level of IgG binding to these LPS fractions was related to the level of IgG anti-GM1 antibody in the serum. Affinity-purified anti-GM1 antibodies showed the same pattern of differential binding to the LPS fractions as the serum from which they were derived. Cholera toxin bound to the same LPS fractions as GBS patients' IgG, the binding of which was blocked by the toxin indicating specific antibody reactivity with a GM1 hapten. The presence of serum anti-GM1 antibodies did not coincide with the presence of the GM1 hapten on the LPS of the infecting strain of Campylobacter indicating that anti-GM1 antibodies do not necessarily arise as part of a simple immune response against the LPS. The IgG antibodies binding to LPS were predominantly of the IgG2 isotype but patients with anti-GM1 IgG had mainly antibodies of IgG1 subclass against both LPS and GM1, implying their production by a T-cell dependent mechanism.

Examples of in vivo isotype class switching in IgM+ chronic lymphocytic leukemia B cells

Chronic lymphocytic leukemia (CLL) usually involves the expansion of a clone of CD5+ B cells synthesizing IgM antibodies. These B cells appear to be blocked at the antigen receptor-expressing stage of B cell differentiation and are thought not to undergo an isotype class switch to IgG or IgA production. In vivo and in vitro studies suggest, however, that in some instances terminal differentiation and isotype switching can occur. To test the hypothesis that in vivo isotype class switching occurs in IgM+ B-type CLL cells, we analyzed the PBMC of 19 CLL patients for the presence of transcripts encoding the rearranged CLL V(H)DJ(H) associated with either gamma or alpha H chains. The molecular data indicate that approximately 50% of B-CLL patients have amplifications of IgM+ B cells that undergo an isotype class switch. Switching to IgA appears to occur more often than to IgG; also, switching can involve different IgG subclasses in individual patients. In many instances, these CLL-related gamma and alpha transcripts are much more plentiful than those of normal B cells that produce the same isotype. These switched transcripts do not reveal evidence for the accumulation of significant numbers of new V(H) gene mutations. The cellular data indicate that B cells with lesser amounts of surface membrane IgD and higher IgM/IgD ratios are more likely to undergo this switching process. Furthermore, B cells expressing IgG and IgA of the same idiotype or V(H) family and the same CDR3 length as those of the CLL IgM+ clone can be identified in the blood of patients studied using multiparameter immunofluorescence analyses. Collectively, these data suggest that not all members of a B-CLL clone are frozen at the surface membrane Ig-expressing stage of B cell maturation, and that some members can switch to the production of non-IgM isotypes. The occurrence of switching without the accumulation of V gene mutations indicates that the processes of differentiation and diversification are not linked.

IgM-producing chronic lymphocytic leukemia cells undergo immunoglobulin isotype-switching without acquiring somatic mutations

The malignant B cells in chronic lymphocytic leukemia (CLL) typically express low-density membrane IgM or IgM/IgD. In vitro experiments have shown that the CLL cells can be induced to differentiate into cells that secrete immunoglobulin (Ig) and can occasionally undergo heavy (H) chain class switching. We now show that the CLL cells also undergo isotype-switching in vivo, since gamma and/or alpha H chain transcripts with identical FW3/CDR3/FW4 regions as the mu CLL transcripts were detected in all of the 13 investigated patients with IgM+ CLL. In most cases switching had occurred to alpha1 and gamma3, but CLL transcripts corresponding to the other gamma chain isotypes were also detected. In one case both the productively and nonproductively rearranged allele were found to undergo H chain class switching. CLL gamma transcripts were also present in surface IgG+ sorted B cells, demonstrating that a small subset of the CLL cells express membrane IgG. In addition, transcripts encoding secretary gamma2 and gamma3 H chains were detected in two cases, which suggests that some serum IgG could be produced by the leukemic clone. Analysis of sorted PBL showed that isotype-switching occurs in CLL cells that express the CD5 antigen. Finally, nucleotide sequence analysis showed that the mu, alpha, and gamma CLL transcripts are identical, demonstrating that the CLL cells do not accumulate somatic mutations in their variable region genes after the H chain class switching. These data provide in vivo evidence that isotype-switching is a frequent phenomenon in CLL, and indicate that a subset of the CLL lymphocytes progress to later stages of B cell differentiation.

Somatic diversification and selection of immunoglobulin heavy and light chain variable region genes in IgG+ CD5+ chronic lymphocytic leukemia B cells

Chronic lymphocytic leukemia (CLL) is characterized by the clonal expansion of CD5-expressing B lymphocytes. Most studies have found that these leukemic CD5+ B cells, like their normal counterparts, use immunoglobulin (Ig) variable (V) region genes that exhibit minimal, if any, somatic diversity. These and other observations have suggested that CD5+ B cells may be incapable of generating Ig V gene diversity, and therefore may not be able to develop higher affinity binding sites that could be selected by antigen. However, most of the studies of CLL and normal CD5+ B cells have focused on IgM-producing cells. Since somatic mutations are most often seen in B cells that have undergone an isotype class switch, we analyzed the Ig heavy (H) and light (L) chain variable region genes of seven IgG+CD5+ CLL B cells to determine if somatic diversification and antigen selection had occurred. The data derived provide evidence for skewed use, somatic diversification, and antigenic selection of the Ig V region genes. Nonrandom use of both H and L chain V region genes was manifested by an overrepresentation of VH4 and VKI family genes and the underrepresentation of the JH4 gene segment. Furthermore, VH4 gene use was restricted to only two family members (4.21 and 4.18). In four of the seven cases, the VH and VL genes displayed > or = 5% difference from the most homologous known germline counterparts. Polymerase chain reaction and Southern blot analyses performed in two of these patients demonstrated that their unique VH CDR2 and adjacent sequences were not present in their germline DNA. In addition, a significant level of diversity was seen in the rearranged DJH segments and at the VL-JL junctions of every patient that occurred both at the time of recombination and subsequently. The localization of replacement changes to complementarity determining regions of some patients suggested that antigen selection had occurred. Furthermore, the mutations identified in the VH and VL genes of each individual patient were strikingly similar, both in number and location. Collectively, the data indicate that a subset of CD5+ CLL B cells can display Ig V region gene mutations. In addition, they are consistent with the notions that in some cases antigen selection of these mutations may have occurred, and that antigen stimulation may be a promoting factor in the evolution of certain CLL clones.