Clinical Significance of Free Circulating CD3 and CD5 in Patients with Chronic Lymphocytic Leukemia.

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CD5, a transmembrane protein expressed in T-cells, few B-cells, and chronic lymphocytic leukemia (CLL) B-cells, is the ligand for CD72 and may play a role in B-cell-T-cell communication. CD5 is part of the T-cell receptor (TCR)-CD3 complex in T-cells as well as the B-cell receptor (BCR) complex and serves as substrate for induction of tyrosine kinase activity. Since leukemic cells have high turnover and pour their protein, RNA, and DNA into the circulation, we speculated that free circulating CD3 (cCD3) and CD5 (cCD5) could be detected in the plasma of patients with CLL. We have developed a bead-based sandwich immunoassay to measure cCD3 and cCD5 in the plasma. Using this assay, we assessed the value of cCD5 measurement, alone and after normalization to cCD3 levels, as a tumor marker in CLL. Plasma levels of cCD3 and cCD5 were measured in 85 patients with CLL and 51 normal control subjects. cCD3 and cCD5 levels were significantly higher in patients with CLL (median, 7,465 and 55,806 U/_μl, respectively) than in normal control subjects (median, 830 and 1,671 U/_μl, respectively). Patients with CLL had significantly higher cCD5:cCD3 ratios (median, 5.28; range, 0–161) than did normal controls (median, 1.70; range, 0–8.06) (P <0.0001). Levels of cCD5, but not cCD3, correlated positively with WBC count, β2-microglobulin level, splenomegaly, and Rai stage (all P <0.01). The cCD5:cCD3 ratio also correlated with Rai stage (P = 0.04) and β2-microglobulin level (P = 0.03). cCD5 levels and the cCD5:cCD3 ratio both correlated with survival (P = 0.03). These findings confirm that free circulating surface markers can be detected in the circulation of patients with CLL, most likely reflect the tumor load, and can be used as tumor markers. The biological and therapeutic relevance of these free circulating proteins should be considered in pharmacokinetic and pharmacodynamic studies.