Lym-1 was one of the first
antibodies to be used successfully for the
radioimmunotherapy of the human
malignant lymphomas. This antibody, which recognizes the
HLA-DR10 antigen preferentially expressed in
B-cell lymphomas, was recently shown to induce apoptosis upon binding to
lymphoma cells. In this study, Lym-1-induced apoptosis was studied to identify the potential molecular pathways of programmed cell death and to demonstrate the clinical potential of this antibody in the treatment of the human
malignant lymphomas. Immunofluorescence microscopy revealed that Lym-1 stained focal areas of the cell surface, consistent with the fact that the
HLA-DR10 antigen is associated with
lipid rafts, a known prerequisite for apoptosis signaling. Likewise,
Annexin V/
propidium iodide staining and TUNEL assays demonstrated that both murine Lym-1 and chimeric Lym-1 induced both early and late apoptosis, respectively, unlike anti-CD20
rituximab. Furthermore, Lym-1 was found to produce a rapid loss of mitochondrial membrane potential and mitochondrial release of
cytochrome C 14 hours post-Lym-1 treatment. Although it was found to activate
caspase-3, inhibitors of
caspase pathways showed that the Lym-1-induced apoptosis in
lymphoma cell lines is independent of
caspase induction. Finally, treatment studies in vivo demonstrated that, compared with murine anti-CD20 (2B8), Lym-1 was more effective in inducing the regression of human
lymphoma xenografts. Based upon these results, chimeric Lym-1 should be especially effective in treating
lymphoma patients, as, in addition to being able to elicit immune effector functions such as chimeric anti-CD20, it can also induce apoptosis directly upon cell binding.