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The expansion of T and natural killer (NK) cells in growth factor-mobilized peripheral blood stem cell (PSC) products with interleukin-2 (IL-2) requires a reduction in monocyte frequency. Monocytes are enriched with stem cells during apheresis and, in this series of growth factor-mobilized PSC products from breast cancer patients, represented 36 ± 6% of the cells in the product. Immunophenotyping studies revealed that monocytes inhibited the proliferation of NK cells (CD56+ and CD3- CDS+ CD56+ cells) and T cells (CD3+, CD4+, and CD8+ cells) during IL-2 co-culture for 7, 14, or 21 days. A reduction in monocytes resulted in 61-fold expansion of CD3- CD8+ CD56+ cells compared with a 3.7-fold increase of CD3+ cells by day 21. In addition, following IL-2 co-culture, cells from PSC products with a reduced frequency of monocytes had a significantly increased T cell mitogenic response and NK cell activity in PSC products compared with intact products. We suggest that monocytes inhibit the IL-2-dependent proliferation and augmented function of NK and T cells from growth factor-mobilized PSC products.