Dominance of Non-CLL Phenotype of Monoclonal B-Cell Lymphocytosis (MBL) in the Middle East and An Overall MBL Prevalence Comparable to Western Countries
Authors: Albitar Maher , Rawas Faisal , Nounou Randa , Bakshi Nasir , Almhareb Fahed , Zahrani Hazzaa Al , Mohamed Said , Rasheed Walid , Khalil Salem , Alsharif Fahad , Chaudhri Naeem A. , El-Solh Hassan , Aljurf Mahmoud 
Published: October 2019

Abstract
Abstract 4586
Chronic lymphocytic leukemia (CLL) is considered more common in Western countries and its incidence is believed to decrease moving east across the globe. CLL is believed to be less common in the Middle East compared to Western countries, although reliable statistical data is not available. Therefore, monoclonal B-cell lymphocytosis (MBL), a pre-CLL condition, is expected to be less common in non-Western countries. In Western countries, MBL prevalence as detected by flow cytometry varies from 0.6% to 14% in healthy individuals older than 40 years, dependent on the level of sensitivity and number of parameters applied. Using 4 to 6 color flow cytometry, most studies report a prevalence of approximately 5 % in the Western population with the CLL-phenotype about 5 times more prevalent than the non-CLL phenotype. MBL incidence and relative proportions of CLL-phenotype versus non-CLL phenotype have not been adequately studied in non-Western countries. We investigated the prevalence and phenotype of MBL in a population sample in the Middle East.

Method:
365 individuals, mostly Saudi Arabian nationals and a smaller number of individuals from neighboring countries, aged over 50 years with normal peripheral blood counts and no evidence of hematologic disease. Peripheral blood samples were immunophenotyped by 8-color flow cytometry detecting CD45, CD19, CD20, CD5, CD10, CD3, kappa and lambda light chains, based on acquiring approximately 1 million cells each.

Result:
Monoclonal B-cells were detected in 21 (6%) individuals (14 male, 7 female, median age 70, range 64–91). However, only 10 cases (48%) displayed the typical CD19+/CD5+ CLL-phenotype. Two cases (9.5%) were CD5-negative clonal B-cells, and 2 (9.5%) were CD10+ clonal B-cells. The remaining 7 cases (33%) showed concomitantly a CD5+ and a CD10+ clonal population, both expressing the same light chain. While we cannot be certain if these CD5+ and CD10+ cell populations represent the same or different clones, the finding that the two populations in all 7 cases showed the same light chain restriction supports the assumption that the two populations represent the same clone.

Conclusion:
MBL in the Middle Eastern region observed in this study is as common as reported in the Western world. In contrast to Western countries, however, it is the non-CLL phenotype which is more prevelant, comprising 52% of our MBL group and most of these cases show cells expressing CD5 and cells expressing CD10. The exact classification of these cases is difficult. It seems likely that these cases represent marginal zone phenotype, but the possibility of a coexisting follicular lymphoma clone cannot be excluded. Pure follicular lymphoma phenotype is seen in 10% of our MBL cases. Further studies with long follow-up are needed.

Disclosures:
No relevant conflicts of interest to declare.