The CD6 scavenger receptor is known to be expressed on virtually all T cells and is supposed to be involved in costimulation, synapse formation, thymic selection and leukocyte migration. Here, we demonstrate that CD6 is differentially expressed by a subpopulation of peripheral CD56^dim natu- ral killer (NK) cells and absent on CD56^bright NK cells. CD56^dimCD16⁺ cells represent the major NK subset in the periphery, and most cells within this group are positive for CD6. Most killer immunoglobulin-like receptor- and immunoglobulin-like transcript-positive cells also belong to the CD6⁺ subpopulation, as expected from their restricted expression on CD56^dim NK cells. In addition, CD6⁺ NK cells are similar to the newly identified CD94^lowCD56^dim NK subpopulation and most distant from the recently defined CD27⁺ NK subpopulation based on the reverse correlation of expression between CD6 and CD27, a marker associated primarily with CD56^bright NK cells. With respect to CD6 function on NK cells, direct CD6 triggering did not result in degranulation but induced secretion of cytokines (interferon-γ and tumor necrosis factor-α) and chemokines [CXCL10 (IP-10), CXCL1 (GRO-α)]. Thus, CD6 expression on peripheral NK cells marks a novel CD56^dim subpopulation associated with distinct patterns of cytokine and chemokine secretion.

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HLA- haploidentical stem cell transplantation (SCT) has been shown to be a suitable alternative for treatment of hematopoietic malignancies if no matched donor is available. Haploidentical family members sharing one HLA-haplotype may differ in one or more HLA-antigens of the second haplotype. The risks are rejection of the graft and severe graft-versus-host disease (GVHD). In particular unmodified grafts may cause severe GVHD, and T-cell depleted grafts may be rejected. We have designed a protocol of unmodified marrow transplantation supplemented by G-CSF mobilized blood cells (MBC) 6 days after marrow transplantation from which CD6-positive cells are depleted. These cells facilitate engraftment and modify GVHD. Patients were conditioned with 12 Gy total body irradiation (TBI), donor leukocytes, antithymocyte globulin and cyclophosphamide, unmodified marrow was given on day 0 and CD6-depleted MBC on day 6. In a first series of 53 patients CD6 cells were depleted by indirect loading and depletion with goat-anti-mouse antibody coupled to Dynabeads. The second series comprised 22 patients whose MBC were depleted using the CliniMacs system with CD6-antibody directly bound to immunomagnetic beads. Table I shows the median and the range () of the cell numbers transplanted after CD6 depletion comparing the two methods. Table II shows the number of patients with acute GVHD in relation to the median of all given transplants. Rejection of the transplant was not observed in 71 evaluable patients. GVHD >= 2 occurred in 47 %, GVHD >= 3 in 21% of 70 evaluable patients. The incidence of acute GVHD was lower in patients given less CD4 cells (p=0.06), it was significantly lower in patients given MBC separated by immunomagnetic beads charged with the specific antibody (p < 0.02). The content of CD8-positive, NK-, B- and CD34-positive cells had no effect on the incidence of GVHD. We conclude from these results that transfusion of CD6-depleted MBC on day 6 facilitates engraftment of marrow from HLA-haploidentical donors without T cell depletion without jeopardizing engraftment and without severe GVHD, if CD4 cells are depleted. A particular advantage is the content of NK cells and CD8 T cells maintaining a potential GVL-effect. Tab.I CD34 x 10e6/kg CD56 x 10e6/kg CD19 x 10e6/kg CD8 x 10e6/kg CD4 x 10e6/kg Indirect labelling (N=53) 7.3 (0.4–35.8) 19.8 (4.8–65.4) 36.1(5.6–117.3) 5.8 (0.75–55) 3.9 (0.12–65.4) Direct labelling (N=22) 13.4(2.5–29.1) 17.1 (1.7–57.3) 48.7 (5.1–175) 2.0 (0.12–13.2) 0.47 (0.05–2.5) Table II aGVHD 0 aGVHD I–II aGVHD III–IV total CD4 <2.29 x 10e6/kg 16 10 4 30 CD4 >2.28 x 10e6/kg 11 16 11 38 CD8 <4.62 x 10e6/kg 11 13 7 31 CD8 >4.61 x 10e6/kg 16 13 8 37