A rationale for positive selection of peripheral blood stem cells in multiple myeloma:: highly purified CD34+ cell fractions of leukapheresis products do not contain malignant cells

In multiple myeloma (MM), the presence of tumor cells in leukapheresis products (LP) has been demonstrated with highly sensitive molecular biological tools in up to 100% of cases. Therefore methods to reduce the tumor load of LP by CD34(+) selection are envisaged. However, there is controversy as to whether the CD34(+) cell is already involved in the malignant process. We have established a PCR assay with allele-specific oligonucleotide primers (ASO) complementary to the CDR3-hypervariable region of the immunoglobulin heavy chain gene of each patient's myeloma clone. Using this ASO-PCR, 43 LP of 10 patients with MM eligible for high-dose therapy were assessed for malignant cells. Furthermore, in an experimental setting we have examined 10 CD34(+) and four CD19(+) fractions obtained from PCR-positive LP by sequential preparative magnetic and fluorescence activated cell sorting (purity >96%) for the presence of the tumor-specific CDR3 region. The majority of LP harbored cells of the myeloma clone (93%), while all CD34(+) fractions were PCR-negative. In all CD19(+) fractions malignant cells were detected. These results confirm that CD34(+) selection can be considered for LP in MM. The sensitivity of the ASO-PCR (up to 10(-5)) enables us further to monitor the efficacy of CD34(+) enrichment protocols in the clinical setting.