Introduction: CD19 is a transmembrane protein restricted to B-lineage cells and follicular dendritic cells. In conjunction with CD21, CD81, and CD225, CD19 acts as a B cell co-receptor to decrease the threshold for antigen receptor-dependent stimulation. CD19 trafficking and expression are orchestrated by a multitude of molecules. Resistance to blinatumomab (Blina, anti-CD19 therapy) in acute lymphoblastic leukemia (ALL) is thought to be due in part to the emergence of CD19-negative escape variants. Loss of expression of CD81 and/or CD21, which are required for CD19 trafficking, results in the lack of CD19 expression. In addition, alternative CD19 splicing and other non-CD19-mediated resistance mechanisms have been described. Therefore, we aimed to study CD19 trafficking and response to Blina in ALL patients.

Methods: Peripheral blood was collected at our Center. Mononuclear cells (PBMCs) were isolated and viably frozen. Cells were then thawed and washed with PBS with 10% FBS, and viable cells were counted using trypan blue dye exclusion. 1x10^6 cells/test were stained for CD19, CD81 (BD Biosciences, San Jose, CA) and CD21 (Biolegend, San Diego, CA). After incubation, cells were washed 2x with FACs buffer, resuspended and acquired on a BD Canto II flow cytometer.

Results: Blood was obtained from 10 patients with relapsed or refractory pre-B-ALL with blood blasts prior to Blina treatment. Five patients subsequently achieved complete remission (CR). Cells from four of these patients had CD21 expression on <5% of cells, but CD81 expression on >40% of cells. Of the 5 patients whose disease did not respond to Blina, four also had CD21 expression on <5% of cells and had CD81 expression on an average of 40% of cells. CD19 was expressed on >40% of cells in all samples.

Conclusion: Although CD21 and CD81 are required for CD19 trafficking and expression, loss of either of these antigens measured by flow cytometry did not correlate with the response to Blina.

Patient 

Age

(years)

 
Diagnosis Karyotype CD19* CD21* CD81* Response to Blinatumomab 
76 Pre-B-ALL 46,XX,t(9;22)(q34.1;q11.2) 53 41 Yes (CR) 
55 Pre-B-ALL 46, X,inv(Y)(p11.2q11.23), dic(9;12)(p13;p13), del20(q11.2) 42 46 Yes (CR) 
47 Pre-B-ALL 46, XX,t(4;11)(q21;q23) 94 12 56 Yes (CR) 
84 Pre-B-ALL 46,XX,i(9)Iq10) 65 72 Yes (CR) 
51 Pre-B-ALL 46, XX, t(9;22)(q34.1;q11.2) 77 78 Yes (CR) 
33 Pre-B-ALL 46, XY, +X, der(17;18)(q10;q10) 67 85 No 
25 Pre-B-ALL 46, XY, der(20;21)(q10;q10),+21 46 32 No 
25 Pre-B-ALL 46, Y, t(X;4)(p11.2;p15), der(9)t(9;14)(p21;q12) del(9)(p21),del(14)q13), +mar[cp14] 74 24 29 No 
79 Pre-B-ALL 46, XY 48 23 No 
10 73 Pre-B-ALL 46, XY 59 29 No 
Patient 

Age

(years)

 
Diagnosis Karyotype CD19* CD21* CD81* Response to Blinatumomab 
76 Pre-B-ALL 46,XX,t(9;22)(q34.1;q11.2) 53 41 Yes (CR) 
55 Pre-B-ALL 46, X,inv(Y)(p11.2q11.23), dic(9;12)(p13;p13), del20(q11.2) 42 46 Yes (CR) 
47 Pre-B-ALL 46, XX,t(4;11)(q21;q23) 94 12 56 Yes (CR) 
84 Pre-B-ALL 46,XX,i(9)Iq10) 65 72 Yes (CR) 
51 Pre-B-ALL 46, XX, t(9;22)(q34.1;q11.2) 77 78 Yes (CR) 
33 Pre-B-ALL 46, XY, +X, der(17;18)(q10;q10) 67 85 No 
25 Pre-B-ALL 46, XY, der(20;21)(q10;q10),+21 46 32 No 
25 Pre-B-ALL 46, Y, t(X;4)(p11.2;p15), der(9)t(9;14)(p21;q12) del(9)(p21),del(14)q13), +mar[cp14] 74 24 29 No 
79 Pre-B-ALL 46, XY 48 23 No 
10 73 Pre-B-ALL 46, XY 59 29 No 

*% cells; CR = complete remission

Note: This abstract was not presented at the meeting.

Citation Format: Firas El Chaer, Brandon Carter-Cooper, Rena G. Lapidus, Maria R. Baer, Zeba N. Singh, Ashkan Emadi. Anti-CD19 therapy for precursor B-acute lymphoblastic leukemia: Response and resistance mechanisms [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 542.