To complete this series on CD38, here are some outstanding questions and answers.
Why do the numbers of CD38-expressing cells in a CLL clone predict clinical course?
A definite answer to this question is not available at this point; the following is the view of the authors on this issue. CD38 expression is dynamic and indicates the proliferative activity of members of the leukemic clone at the time of analysis. Therefore, it is a “real-time” indicator of the level of leukemic proliferation and thereby actual or potential clonal evolution, which ultimately determines the clinical course and outcome for an individual patient. This evolutionary change can be affected by many parameters; in particular, it can be influenced by stimulation through cell surface receptors for antigens, cytokines, chemokines, etc, within the microenvironment. Simplistically viewed, the more CD38+ cells in a clone, the greater the number of dividing cells and hence the greater the chance for occurrence of new DNA lesions, enhanced clonal aggressiveness, and worse clinical outcome.
Why and how do the numbers of CD38-expressing cells and IGHV mutation status of a CLL clone interrelate and herald clinical course?
IGHV mutation status is a more static marker, indicative of the cell of origin and the maturational events that occurred in the life of the B cell before its leukemic transformation. In particular, the absence or presence of IGHV mutations could report the ability and type of antigen binding that the BCR can accomplish (eg, polyreactive vs oligoreactive vs monoreactive). Antigen binding specificity correlates with outcome because it indicates the breadth of antigenic epitopes that the BCR can engage, the affinity of these engagements, and therefore the likelihood that survival/proliferation signals will be delivered to the CLL cell. It is for these reasons that CD38 expression, which is a reflection of the existing level of cellular activation within a leukemic clone, often correlates with a lack of IGHV somatic mutations, which is more likely to lead to polyspecific binding, cell signaling, and eventual cellular proliferation. Thus, both prognostic indicators can be linked by the common thread of leukemic cell proliferation: IGHV mutations indicating the likelihood of binding multiple antigens and of cellular stimulation and CD38 expression representing the consequence of such binding and stimulation.
What are the advantages of using CD38 clonal percentages as a prognostic marker?
As mentioned, the number of CD38-expressing cells can be considered a real-time indicator of the proliferative activity of a patient's CLL clone. As such, it is a reflection or harbinger of new genomic lesions, which require DNA replication and subsequently of clonal evolution to a more dangerous leukemic variant.
What are the disadvantages of using CD38 clonal percentages as a prognostic marker?
It has been documented that CD38 levels can change over time. Although this change is usually not large (∼ 10%) and more often than not does not overstep the boundaries that mark “better” or “worse” clinical outcome, it can occur. However, when it does, the suggested downside can be actually viewed as a positive feature of this prognostic indicator as an upward trend in CD38-expressing cells may signal clonal evolution to a more aggressive state. Therefore, serial analyses of the percentage of CD38+ cells, real-time indicators of leukemic cell proliferation, can have an additional advantage of indicating a change in clonal behavior.
Is there a role for CD38 SNP analysis?
The C > G SNP identified in the regulatory region is localized within an E-box and conditions the affinity of E2A binding. The consequence is that G carriers up-regulate CD38 in response to environmental signals with a higher efficiency than CC homozygotes. It would be of interest to test, in a retrospective cohort, whether the C > G SNP affects the stability of CD38 expression over time. The prediction based on in vitro observations would be that G carriers up-regulate CD38 more readily in response to specific external signals and then become CD38+. CC homozygotes, on the other hand, would be less prone to do so. If confirmed, this observation would support the clinical usefulness of identifying the CD38 SNP at diagnosis, with closer monitoring of CD38 expression over time selectively in G carriers. In addition, clinical testing for the G allele, one of the few risk factors for RS transformation, may highlight patients more susceptible to this dangerous event.
How can CD38 expression be best used as a prognostic indicator?
The current National Cancer Institute guidelines recommend that treatment be initiated at disease progression (ie, in patients at Binet stage C or Rai stage III or IV). This strategy is suggested because patients are heterogeneous in clinical course and certain patients never progress, often dying of a different disease, whereas others do progress, at times quite rapidly. Thus, to prevent unnecessary treatment of a large segment of affected persons, a watch-and-wait attitude is chosen and treatment is generally delayed until patients become symptomatic. The disadvantage of this understandably prudent approach is that treatment may come too late for those persons requiring it, when a number of unfavorable cytogenetic lesions have already accumulated in CLL subclones. Therefore, the option of early treatment should be carefully considered if markers were available to precisely predict clinical course and progression to select patients based on real treatment requirement. So far, none of the available predictors alone is sufficiently precise to mandate a change in the present therapeutic strategy. In addition, most of the available studies with marker combinations have been carried out retrospectively and in studies where the precise assessment of the value of CD38 can be hampered by the time of marker determination during the clinical course, perhaps because of change that can occur longitudinally.
Despite these limitations, scoring systems involving a multiplicity of markers (eg, IGHV mutations), CD38 and/or ZAP-70 levels, chromosomal abnormalities, p53 mutations, and serum molecules such as β2-microglobulin, can be effective risk assessors and group stratifiers. The issue of markers is covered by a wide literature. These approaches may require further prospective testing to refine and simplify the methods. At this point, the watch-and-wait strategy should be compared with that of early, potentially aggressive treatments in patients randomized into a formal clinical trial and selected for expression of unfavorable prognostic markers.
Is there a role for CD38 as a therapeutic target?
Targeted immunotherapy with mAbs has become critical for the successful treatment of many forms of cancer. This is exemplified by rituximab, a chimeric anti-CD20 mAb, which has revolutionized the treatment of several B-cell malignancies. The main concern about in vivo use of anti-CD38 mAbs is its widespread expression in multiple cell types and differentiation stages, such as early committed BM precursors, activated T cells, and cells of the innate immune response. Another serious drawback is the presence of CD38 in the brain, pancreas, and retina, although the earliest hematopoietic stem cells do not appear to express CD38. Another potentially useful approach could rely on the use of inhibitors of the enzymatic activities of CD38, even if experience is so far limited.
These intrinsic limits, however, have not prevented the design of models for in vivo applications. In the past, several Abs to human CD38 have been generated that induce killing of neoplastic B-cell lines. Two CD38 mAbs are currently in clinical development: a humanized mAb (SAR650984, http://clinicaltrialsfeeds.org/clinical-trials/show/NCT01084252) and a human mAb (daratumumab). Ongoing clinical trials will determine whether these reagents are effective (alone or in combination) and at the same time safe.