Biological and Clinical Significance of CD81 Expression by Clonal Plasma Cells in High-Risk Smoldering and Symptomatic Multiple Myeloma (MM) Patients,

Abstract 3936

The presence of CD19 in myelomatous plasma cells (MM-PC) correlates with adverse prognosis in MM. Although CD19 expression is up-regulated by CD81, this marker has been poorly investigated and its prognostic value in myeloma remains unknown.

Herein, we assessed the frequency and the prognostic value of the immunophenotypic detection of CD81 surface expression in MM-PC of high-risk smoldering (SMM) and symptomatic MM patients at diagnosis and its role as a potential therapeutic target. The study included 230 elderly MM patients treated according to the Spanish GEM05>65y trial, and a validation set based on 325 transplant candidates MM patients enrolled in the GEM05<65y trial was used. In addition we analyzed a total of 56 high-risk SMM patients. The median follow-up was 32 and 22 months for the MM and SMM, respectively. Expression of CD81 on MM-PC was assessed by multiparameter flow cytometry (MFC). FISH was performed at baseline in immunomagnetic-enriched PCs from 211 of the 230 elderly MM patients, and in a subset of these patients (N=23) mRNA gene expression profiling (GEP) was also performed.

MFC immunophenotyping showed the presence of CD81⁺ MM-PC in 20 of 56 (36%) SMM, and 90 of 230 (39%) MM patients. CD81⁺ SMM cases had a shorter TTP to symptomatic disease than CD81⁻ patients (NR vs 37 months, P =.02). Similarly, CD81⁺symptomatic MM patients showed shorter PFS (3y: 29% vs 48%, P <.001) and OS (3y: 64% vs 76%, P =.008) rates compared to CD81⁻ cases. Multivariate analysis including other baseline variables showed that the best combination of independent predictive parameters for PFS were: CD81⁺ expression on MM-PC (HR=1.8; P =.004), high-risk cytogenetics (HR=1.8; P =.02) and percentage of MM-PC in S-phase (>2%; HR=1.7; P =.02); in turn for OS, CD81⁺ expression (HR=2.2; P =.005), high-risk cytogenetics (HR=2.2; P =.006) and age (≥75 years; HR=1.8; P =.03) were selected. To validate the adverse impact of CD81⁺ expression, we explored whether this new marker would retain its prognostic value in a series of 325 transplant candidates, symptomatic MM patients. CD81⁺ cases (138 out of 325, 42%) showed shorter PFS (3y: 44% vs 62%, P <.001) and OS (3y: 74% vs 89%, P =.004) rates as compared to CD81⁻ cases.

The prognostic influence of CD81 expression prompted us to investigate its potential role as a therapeutic target in MM cell lines. 5 out of the 13 cell lines analyzed were homogeneously positive for CD81 (RPMI-8226, RPMI-LR5, NCI-H929, OPM-2, JJN3) while the others exhibited no expression; these results were further validated by western blotting. We then tested the effect of two different anti-CD81 antibodies on cell proliferation on 2 CD81⁺ cell lines (RPMI-8226 and JJN3) as well as in 1 CD81⁻ (MM1S) MM cell line. Interestingly, the two antibodies tested decreased cell proliferation (around 30%, P ≤.005) in the two CD81⁺ cell lines, whereas no differences were found for the CD81⁻ MM1S cell line. To assess whether anti-CD81 antibodies facilitate immune effector cell function, we performed in vitro ADCC. However, no effect was noted in the CD81⁺ RPMI-8226 cell line, using either PBMCs or the macrophage cell line RAW264.7, and at different cell ratios. Similar results were also found upon using CDC assay in CD81⁺ RPMI-8226 and JJN3 cell lines.

Finally, we explored in a subgroup of MM patients (n=23) in which information was available, the relationship between positivity for CD81 by MFC and its genomic expression. The expression level of CD81 mRNA was significantly decreased in CD81⁻ MM cases (P <.001) vs. both healthy adults and CD81⁺ patients (P <.001), with no significant differences between these two latter subgroups. Moreover, we found a highly significant correlation between the expression of CD81 mRNA by GEP and the percentage of CD81⁺ MM-PC by MFC (r=.812; P <.001). We further investigated by GEP, differences in other genes involved in the CD81 signaling pathway, and CD81⁺MM patients showed significantly higher levels of CD79A (P =.03) and SYK (P =.02) mRNA than CD81⁻cases.

In summary, our findings show the existence of a phenotypic-genomic correlation of CD81 expression in patients with myeloma. Expression of CD81 in MM-PC is an independent prognostic factor for patients with symptomatic MM and a marker for risk of progression in SMM. Blockage of CD81 with anti-CD81 antibodies does not show significant anti-myeloma activity; therefore, the precise mechanism of CD81 activation of MM-PC deserves further investigations.