CD38 Expression in B-Lineage Acute Lymphoblastic Leukemia, a Possible Target for Immunotherapy

Introduction:

CD38 is a type II transmembrane glycoprotein expressed on immature T and B lymphocytes (thymocytes and hematogones), NK cells, activated T-cells, plasma-cells, monocytes and red blood cells. This ectoenzyme is a ribonucleosyl cyclase (Cyclic ADP ribose hydrolase) involved in the regulation of calcium fluxes.

CD38 is absent from quiescent lymphocytes. The lowest levels are present on erythrocytes while CD38 is brightly expressed by hematogones and plasma-cells. The latter has led to the development of daratumumab, a therapeutic monoclonal antibody used in the treatment of multiple myeloma.

Because of the wide distribution of this antigen, other diseases could be considered for such a therapy. Among them, acute lymphoblastic leukemia (ALL) could represent an interesting target.

We thus investigated the level of expression of CD38 in a cohort of 128 samples from B-lineage ALL (113 at diagnosis and 15 at relapse). We also compared it to that of the normal counterpart of these blasts, hematogones, because of the potential use of CD38 as a leukemia associated immunophenotype (LAIP). Moreover, in 15 paired samples of diagnosis/relapse, we examined the stability of this expression during disease evolution.

Patients, Material and Methods:

A total of 62 females and 66 males were included in the cohort, with a median age of 20 years old (range 4 months to 90 years). There were 57 children (below 15 yo), 13 adolescents and young adults (15-25 yo) and 58 adults. CD38 expression was investigated in 45 bone marrow (BM) samples and in 83 peripheral blood (PB) samples. The median level of blasts was 62% in BM and 48% in PB. According to EGIL classification, there were 19 B-I, 66 B-II, 38 B-III, 2 B-IV and intracytoplasmic mu chain was not investigated in 3 cases.

In parallel, 26 samples of BM with morphologically evidence hematogones were used to compare the level of expression of CD38 on these cells. Immunophenotyping panels comprised CD38 antibodies conjugated to allophycocyanin and all samples were analyzed on a Canto II flow cytometer (BD biosciences, San Jose, CA).

Results:

CD38 was always present on hematogones. It was expressed by 122 of the 128 B-ALL samples tested (95,3%). Partial expression, between 20 and 70% of the blasts was noted for 9 patients (7%) while 113 patients (88%) had more than 70% positive blasts. For 102 patients (79,7%), the whole population was stained (100% of the blasts). Among the 15 samples obtained at relapse, CD38 was always expressed, partially only in one case.

The mean fluorescence intensity (MFI), expressed using the flow cytometer arbitrary units (AI) ranged between 510 and 1396 AI (median 878 AI) in the group of patients with 20-70% CD38+ blasts (n=9) and between 317 and 26466 AI (median 4704 AI) for the 113 patients with more than 70% CD38+ blasts (n=113).

The level of fluorescence was always higher on hematogones compared to blasts, with a median MFI of 14915 AI (range 3834-34501 AI). CD38 expression is also used as a LAIP, to discriminate minimal residual disease (MRD) from regenerating hematogones. In our department, a threshold of 6300 AI (median of hematogones - 2 SD) is used to define the LAIP of CD38^low blasts. In the cohort reported here, 82 patients presented this LAIP (64%). Conversely, high expression of CD38 with an MFI >10 000 AI was present for 23 of the 128 patients.

For the 15 patients with paired samples of diagnosis and relapse, 9 had low levels of CD38 (LAIP). In none of these cases modulation of CD38 was observed, this LAIP remaining stable over time.

Conclusion:

CD38 has been reported by several authors as one of the best LAIP marker for the detection of MRD in B-ALL, allowing a good follow-up of the patients because of its stability. Moreover, this study confirms that CD38 could be a valuable therapeutic target in most of B-ALL cases, being expressed in over 95% of the cases. Used together with chemotherapy, daratumumab could thus be useful to treat B-ALL both in first line or at relapse.