Abstract 542: Anti-CD19 therapy for precursor B-acute lymphoblastic leukemia: Response and resistance mechanisms
Authors: Chaer Firas El , Carter-Cooper Brandon , Lapidus Rena G. , Baer Maria R. , Singh Zeba N. , Emadi Ashkan 
Published: August 2019

Abstract
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 &lt;5% of cells, but CD81 expression on &gt;40% of cells. Of the 5 patients whose disease did not respond to Blina, four also had CD21 expression on &lt;5% of cells and had CD81 expression on an average of 40% of cells. CD19 was expressed on &gt;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.
PatientAge(years)DiagnosisKaryotypeCD19*CD21*CD81*Response to Blinatumomab176Pre-B-ALL46,XX,t(9;22)(q34.1;q11.2)53241Yes (CR)255Pre-B-ALL46, X,inv(Y)(p11.2q11.23), dic(9;12)(p13;p13), del20(q11.2)42146Yes (CR)347Pre-B-ALL46, XX,t(4;11)(q21;q23)941256Yes (CR)484Pre-B-ALL46,XX,i(9)Iq10)65372Yes (CR)551Pre-B-ALL46, XX, t(9;22)(q34.1;q11.2)77578Yes (CR)633Pre-B-ALL46, XY, +X, der(17;18)(q10;q10)67185No725Pre-B-ALL46, XY, der(20;21)(q10;q10),+2146432No825Pre-B-ALL46, Y, t(X;4)(p11.2;p15), der(9)t(9;14)(p21;q12) del(9)(p21),del(14)q13), +mar[cp14]742429No979Pre-B-ALL46, XY48123No1073Pre-B-ALL46, XY59129No*% 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.