Donor lymphocyte infusion (DLI) reliably induces durable remission in 75-80% of patients with relapsed
chronic myelogenous leukemia (CML) after allogeneic
hematopoietic stem cell transplantation. To identify immunological targets of the graft-versus-
leukemia response (GVL) after DLI, we used CML post-DLI responder sera to screen a CML
cDNA expression library. One of the
antigens identified in this screen is a M(r) 28,000
protein, termed CML28. CML28 is identical to hRrp46p, a component of the human exosome, a multiprotein complex involved in the 3' processing of
RNA. Components of the human exosome include known
autoantigens, such as PMScl-100, an
autoantibody target in patients with
polymyositis, scleroderma, or
polymyositis-scleroderma overlap syndrome. Recombinant CML28-GST fusion
protein was purified, and used in Western blot and ELISA to demonstrate the development of a high-titer CML28-specific
IgG antibody response in a patient with relapsed CML who responded to DLI. Northern blotting demonstrated that CML28 is highly expressed in a variety of hematopoietic and epithelial tumor cell lines, but not in normal hematopoietic tissues or other normal tissue, with the exception of testis. Purified recombinant CML28 was used to generate a CML28-specific murine
monoclonal antibody. Western blotting with CML28
monoclonal antibody against whole-cell lysates derived from blood and marrow of normal donors and patients with
leukemia revealed high expression of this
antigen in
tumor but not in normal samples. Because CML28 was highly expressed in epithelial tumor cell lines, anti-CML28 responses were also examined in patients with solid
tumors. By ELISA, we found specific serological responses in 10-33% of patients with
lung cancer,
melanoma, and
prostate cancer. Our studies suggest that immunogenicity of CML28 is likely because of overexpression of this
antigen in
tumor cells. Moreover, given its expression and immunogenicity in a wide variety of
malignancies, CML28 merits additional evaluation as a target for
antigen-specific
immunotherapy.