High Expression of CD40 on BCP-ALL Blasts Is an Independent Risk Factor Associated with Improved Survival and Enhanced Capacity To Upregulate the Death Receptor CD95.

Leukemogenesis is facilitated by deregulated survival signals and escape from immunosurveillance. For immunological control both adequate stimulation of effector cells as well as apoptosis sensitivity of target cells are required. The CD40 receptor, a member of the TNF/NGF receptor family, plays a pivotal role on normal B cells in both aspects as it upregulates the costimulatory molecules enhancing their antigen presenting capacity and the CD95 TNF receptor sensitizing cells to apoptotic stimuli. Due to its physiological immunomodulatory functions, we prospectively studied the prognostic impact of CD40 expression on primary precursor B (BCP)-ALL blasts of pediatric patients (pts) (n=121). Blasts exhibited an immunephenotype of c-ALL (n=81), preB-ALL (n=32) or proB-ALL (n=8). All pts (>1 and <17y) were treated according to the CoALL06-97 study protocol. The criteria for prognostic classification into high risk (HR; n=57) and low risk (LR; n=64) pts at initial diagnosis include immunephenotype, leukocyte count (WBC), age, genetic translocations and the in vitro resistance to chemotherapeutic agents (PVA score). In flow cytometry analysis blasts exhibited a broad range of CD40 expression with a mean of 80%±27% (±SD) positive blasts. In terms of the above mentioned prognostic discrimination criteria, we found significantly lower percentage of CD40 positive blasts in HR pts (median; range: 84%;3–100%) compared to the LR group (98%;3–100%) (p=0.007). With regard to prognostic relevance, pts with very high percentage (above the mean of 80%) CD40 positive blasts exhibited significantly better relapse free survival (RFS; 0.86±0.06 vs 0.65±0.09; p=0.009) in Kaplan Meier analysis and multivariate Cox regression analysis revealed that high CD40 expression was independently associated with superior RFS in our pediatric ALL pt cohort (p=0.006) in contrast to the risk factors age (p=0.72), WBC (p=0.33) and PVA score (p=0.60). In a subset of patients (n=34) we examined whether the level of CD40 expression corresponds to the capacity of blasts to upregulate costimulatory molecules and the CD95 death receptor upon CD40-ligation. CD40-crosslinking in samples with a very high percentage of CD40 positive blasts (>80%) and with intermediate/low expression of CD40 (<80%) did not result in significantly different upregulation of CD80 (37%;3–97% vs 41%;4–68%) and CD86 (83%;5–100% vs 72%;12–96%) in the two sample-sets (p=0.66 and p=0.86). In contrast, expression of CD95 in CD40-stimulated blasts showed a significant correlation with CD40 baseline expression (p=0.009) with a higher percentage of CD95 positive blasts following CD40-activation in samples containing >80% CD40 positive blasts compared to samples with intermediate/low CD40 expression (74%;17–95% vs 38%;9–94%; p=0.065). Thus, high percentage of CD40 positive blasts is an independent prognostic marker that identifies a group of pediatric BCP-ALL pts with favorable outcome. While the capacity to upregulate costimulatory molecules following CD40-activation might contribute to the observed improvement in survival, enhanced upregulation of the CD95 receptor seems to be the leading mechanism mediating sensitization towards apoptotic stimuli.