Patients with chronic lymphocytic leukemia (CLL) cells that express unmutated immunoglobulin (Ig) heavy chain variable region genes (IgVH genes) generally have a more aggressive clinical course than do patients with leukemia cells that express mutated IgVH. The reason(s) accounting for this are not known. Microarray gene expression analyses revealed that CLL cells that express unmutated IgVH could be distinguished from the leukemia cells that express mutated IgVH via the differential expression of a relatively small number of genes, one of which encodes the zeta-associated chain of 70kD (ZAP-70), which generally is expressed by CLL cells that express unmutated IgVH. Although the expression of ZAP-70 is associated with expression of unmutated IgVH in CLL, this association is not absolute. This was the case for a pair of monozygotic twins who both developed CLL at age 57. Although each of the twins had leukemia cells that expressed mutated IgVH, only one of the twins had leukemia cells that lacked expression of ZAP-70 protein and has indolent, non-progressive disease (
). We performed microarray analysis using Affymetrix HG-U133A array on the isolated leukemia cells of each twin to define the genes that were differentially expressed between the two. In addition to ZAP-70, we found that the CLL cells of the twin with progressive disease also expressed the inducible co-stimulatory molecule (ICOS), a member of the CD28/CTLA-4 family of immune accessory co-stimulatory molecules that ordinarily only is expressed by activated T cells. Expression of ICOS protein by this leukemia B cell population, but not by the CLL B cells population of the other twin, was confirmed using fluorochrome-labeled anti-ICOS mAb and flow cytometry. We examined the CLL B cells from 58 additional patients for expression of ICOS by flow cytometry and found that 16 (28%) also expressed ICOS. We found that expression of ICOS was associated with expression of ZAP-70, as assessed via flow cytometry and immunoblot analyses. Whereas 14 of the 29 ZAP-70+ cases expressed ICOS, only 2 of the 29 ZAP-70-negative cases expressed this immune co-stimulatory molecule. Nevertheless, we found that nearly all of the 56 of the 58 cases expressed B7h, the ligand for ICOS. The two cases that did not express detectable B7h expressed ZAP-70 and were ICOS+. In preliminary studies, we found that treatment of ICOS-negative, ZAP-70+ CLL cells (n = 2) with goat anti-human Ig could induce expression of ICOS, suggesting that, as on T cells, this molecule also might be inducible in some cases of B cell CLL. Culture of ICOS+ CLL cells with an anti-B7h mAb capable of blocking ICOS-B7h interactions significantly enhanced ICOS surface expression, as assess by flow cytometry, suggesting that B7h may down-modulate ICOS through paracrine/autocrine receptor-ligand interactions. Because of this we evaluated for functional expression of ICOS on CLL B cells. We found that ligation of ICOS could induce enhanced signaling via the PI3K/Akt pathway in isolated CLL B cells, resulting in enhanced phosphorylation and activation of Akt. As such, we speculate that the expression of ICOS and its ligand in B cell CLL may enhance leukemia cell survival and/or proliferation, potentially contributing to the more aggressive disease observed in some patients with this disease.
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