An important theme in molecular cell biology is the regulation of protein recruitment to the plasma membrane. Fundamental biological processes such as proliferation, differentiation or leukocyte functions are initiated and controlled through the reversible binding of signaling proteins to phosphorylated membrane components. This is mediated by specialized interaction modules, such as SH2 and PH domains. Cytohesin-1 is an intracellular guanine nucleotide exchange factor, which regulates leukocyte adhesion. The activity of cytohesin-1 is controlled by phosphoinositide-dependent membrane recruitment. An interacting protein was identi®ed, the expression of which is upregulated by cytokines in hematopoietic cells. This molecule, CYTIP, is also recruited to the cell cortex by integrin signaling via its PDZ domain. However, stimulation of Jurkat cells with phorbol ester results in re-localization of CYTIP to the cytoplasm, and membrane detachment of cytohesin-1 strictly requires co-expression of CYTIP. Consequently, stimulated adhesion of Jurkat cells to intracellular adhesion molecule-1 is repressed by CYTIP. These ®ndings outline a novel mechanism of signal chain abrogation through sequestration of a limiting component by speci®c protein±protein interactions.

IntroductionSevere immune dysfunction is a predictable consequence of high-dose anticancer chemotherapy with or without autologous or allogeneic stem cell support and results in significant morbidity and mortality in potentially curable diseases. [1][2][3] One obvious explanation for such immune dysfunction is a depletion of immune cells by the cytotoxic effects of anticancer drugs. 4,5 However, even after quantitative regeneration immune cells do not regain full immune competence for a prolonged period of time. [6][7][8][9][10] T cells play a key role in multiple immune responses. Therefore, both a depletion and a disturbed function caused by cytotoxic anticancer treatment are potentially harmful to the immune surveillance of the host, including an increased susceptibility to infections with otherwise less harmful pathogens. [11][12][13][14][15] T-cell reconstitution after cytotoxic therapy does not repeat fetal T-cell ontogeny. [16][17][18][19] In fact, the first T cells that repopulate the host after T-cell depleting (TCD) therapy expand extrathymically from peripheral residual T cells whereas a thymus-dependent T-cell regeneration is delayed and may be even incomplete in adult patients in whom thymic function has declined. [20][21][22] Extrathymically expanded T cells bear a memory phenotype, 23 as their expansion is determined by antigenic stimulation. 24 Their capacity to restore full T-cell function is believed to be limited, because of (1) skewing of the T-cell repertoire 25-27 and (2) an increased susceptibility to apoptosis, including activation-induced apoptosis. 28 In contrast, T cells with a naive phenotype presumably represent recent thymic emigrants, 23,[29][30][31] having matured from precursors via thymopoiesis, and thus may be spared from the postchemotherapy immune dysfunction.To test this hypothesis we investigated by flow cytometry the in vitro immune function of distinct naive (CD4 ϩ CD45RA ϩ ) and memory (CD4 ϩ CD45RA Ϫ ) T-helper cells. Examinations were performed at one to 3 months after completion of TCD therapy, a time point at which CD4 ϩ CD45RA ϩ T cells were readily detectable but not yet fully recovered.Upon stimulation with polyclonal mitogens in vitro we found a reduced up-regulation of the early activation-induced cell surface molecule, CD69, detectable in CD4 ϩ T cells even in the presence of exogenous interleukin 2 (IL-2). Strikingly, CD4 ϩ CD45RA ϩ cells were similarly affected as CD4 ϩ CD45RA Ϫ cells by decreased stimulatory responses and an increased susceptibility to apoptosis as detected by annexin V staining. 32,33 When peripheral blood mononuclear cell (PBMC) populations were enriched for T cells, the stimulatory responses improved markedly. Together, the findings are consistent with a concept that factors external to T cells, eventually affecting T-helper cells irrespective of a naive or memory phenotype, contribute to the prevailing T-cell dysfunction in the early post-TCD therapy period. Patients, materials, and methods PatientsT-cell analyses in pediatric patients ...