Characterization of CD22 expression in acute lymphoblastic leukemia

Abstract

Background: CD22 is a B-lineage differentiation antigen that has emerged as a leading therapeutic target in acute lymphoblastic leukemia (ALL).

Procedure: Properties of CD22 expression relevant to therapeutic targeting were characterized in primary samples obtained from children and young adults with relapsed and chemotherapy refractory B-precursor (pre-B) ALL.

Results: CD22 expression was demonstrated in all subjects (n = 163) with detection on at least 90% of blasts in 155 cases. Median antigen site density of surface CD22 was 3,470 sites/cell (range 349-19,653, n = 160). Blasts from patients with known 11q23 (MLL) rearrangement had lower site density (median 1,590 sites/cell, range 349-3,624, n = 20 versus 3,853 sites/cell, range 451-19,653, n = 140; P = <0.0001) and 6 of 21 cases had sub-populations of blasts lacking CD22 expression (22%-82% CD22 +). CD22 expression was maintained in serial studies of 73 subjects, including those treated with anti-CD22 targeted therapy. The levels of soluble CD22 in blood and marrow by ELISA were low and not expected to influence the pharmacokinetics of anti-CD22 directed agents.

Conclusions: These characteristics make CD22 an excellent potential therapeutic target in patients with relapsed and chemotherapy-refractory ALL, although cases with MLL rearrangement require close study to exclude the presence of a CD22-negative blast population.

Keywords: CD22; acute lymphoblastic leukemia; monoclonal antibody; relapse.

Figure 1. CD22 Expression in Representative Samples

Quantification of CD22 expression (antigen binding capacity [ABC] value) by flow cytometry using QuantiBRITE method (BD Biosciences) A. Gating strategy for analysis of ALL blasts. CD10+ CD19+ cells analyzed. B. ALL with partial CD22 positivity (MLL-rearranged). C. ALL with CD22 ABC = 1,063. D. ALL with CD22 ABC = 3,045. E. ALL with CD22 ABC = 10,392.

Figure 2. CD22 Antigen Site Density on Patient Lymphoblasts

Quantification of CD22 expression on patient blasts was conducted using the QuantiBRITE method (BD Biosciences). Median density for subjects with known MLL rearrangement was 1,590 sites/blasts (range, 349-3,624). Median site density for those without MLL rearrangement was 3,470 sites/blasts (range, 451-19,653).

Figure 3. CD22 Expression Before and After Anti-CD22 Targeted Therapy

A. Quantification of CD22 expression on blasts from 44 non-MLL rearranged patients before and after treatment with anti-CD22 targeted therapy demonstrates no loss of CD22 expression following CD22 targeted therapy (p=0.07). Median CD22 site density pre-therapy was 3,080 (range, 620-14,519 sites/cell) and post-therapy was 2,972 (range, 549-15,392 sites/cell). B. Quantification of serial CD22 expression from 14 subjects who received anti-CD22 targeted therapy (≥ 5 samples/patient over a 2-year period). Maximum number of screens/patient=10

Figure 4. Emergence of a CD22 Dim Population in a Patient with MLL-Rearranged ALL Treated with Anti-CD22 Targeted Therapy

A 1 year old with MLL-rearranged ALL was treated with an anti-CD22 immunotoxin. A subpopulation of CD22 dim blasts was detectable on the pre-treatment sample (median site density 1,643) (A). An initial dramatic reduction in circulating lymphoblasts was followed by emergence of a CD22 dim blast population (median site density 605) (B) despite ongoing therapy.