c-kit is not expressed in Hodgkin disease and anaplastic lymphoma kinase (ALK)-positive anaplastic large cell lymphoma

c-kit receptor (CD117) is expressed by erythroid, megakaryocytic, and myeloid precursors and mature mast cells.¹  Imatinib mesylate (STI571), a well-established inhibitor of bcr-abl protein tyrosine kinase and currently used for the treatment of chronic myeloid leukemia,²  also inhibits c-kit receptor kinase activity.³ 

A previous study in Blood reported that c-kit is expressed in most cases of anaplastic large cell lymphoma (ALCL) and approximately half of Hodgkin disease (HD) tumors, but not in other low- or high-grade non-Hodgkin lymphomas.⁴  c-kit has also been reported to be expressed in most HD cell lines.⁵  In view of the possible use of STI571 as an experimental treatment for these lymphomas, we assessed c-kit expression at the RNA and protein levels in 5 known classical HD cell lines (HD-MYZ, HDLM2, L-428, KM-H2, and L-1236; purchased from DSMZ, Braunschweig, Germany) and 5 ALK-positive ALCL cell lines, including Karpas 299, SR-786, and SU-DHL-1 (DSMZ) and JB-6 and TS-G1 (a gift from Dr. D. Jones, Houston, TX). The gastrointestinal stromal tumor cell line ST-882 (a gift from Dr J. Trent, Houston, TX) and the megakaryoblastic leukemia cell line MO7e (a gift from Dr M. Andreeff, Houston, TX) served as positive controls for c-kit expression. Using standard reverse transcriptase-polymerase chain reaction (RT-PCR), flow cytometry (monoclonal antibody 104D2; BD Biosciences Pharmingen, San Diego, CA), and Western blot analysis (polyclonal antibodies from DAKO, Carpinteria, CA, and Santa Cruz Biotechnology, Santa Cruz, CA), no evidence of c-kit RNA or protein was detected in any of the HD and ALCL cell lines (Figure 1A). These results were further confirmed by immunohistochemistry using formalin-fixed, paraffin-embedded cell blocks prepared from these cell lines.

c-kit expression was also immunohistochemically assessed in 52 HD (47 classical HD, 5 lymphocyte-predominant HD) and 22 ALK-positive ALCL tumors using methods described elsewhere⁶  and a polyclonal antibody (DAKO) reactive with c-kit that is reported to yield optimal immunohistochemical results.⁷  All HD and ALK-positive ALCL tumors were negative for c-kit(Figure 1B). Our data are in agreement with 2 recent reports that demonstrated no evidence of c-kit immunoreactivity in a total of 32 HD tumors.^8,9  These results and our data are contrary to those of Pinto and coworkers,⁴  published in Blood, who found c-kit expression in fresh-frozen sections of 11 of 16 ALCL and 11 of 21 HD tumors. Subsequently, the same group reported that 5 of 6 HD cell lines expressed a functional c-kit receptor.⁵  In both studies, the authors used the 17F11 monoclonal antibody that recognizes an extracellular domain of c-kit. The 17F11 antibody was raised by immunization of a Balb/c mouse with leukemic blasts from a patient with acute nonlymphocytic leukemia and is reported to react positively with most leukemic blasts of myeloid, but not lymphoid, lineage or peripheral blood cells¹⁰  Since the 17F11 antibody is no longer commercially available, we are not able to test its immunoreactivity in this study.

The explanation for the remarkable discrepancy among previous studies regarding c-kit expression in HD and ALCL is uncertain. However, the results obtained using the 17F11 antibody have not been confirmed using other commercially available antibodies.^8,9 

We conclude that c-kit RNA and protein are consistently not expressed in HD and ALK-positive ALCL cell lines and tumors. These findings suggest that c-kit tyrosine kinase is probably not an appropriate target for therapeutic agents such as STI571 in the treatment of patients with HD and ALCL.