Thymus-dependent T-cell regeneration is a major pathway for immune reconstitution after stem cell transplantation in children. Therefore, we prospectively assessed T-cell dynamics and thymic function in 164 pediatric patients between 1 and 124 months after transplantation by measuring T-cell receptor recombination excision circles and spontaneous expression of Ki67 in peripheral T-cell subsets. We analyzed the effect of recipient age, conditioning regimen, type of donor and graft, stem cell dose, and graft-versus-host disease on the onset and the plateau of thymic output. A high rate of spontaneous proliferation in early-reconstituting naive and memory T cells inversely correlated with total T-cell numbers. Accordingly, T-cell receptor recombination excision circle content was diminished in early-appearing naive T cells. A multivariate analysis revealed that the onset of thymic recovery was inversely correlated only with recipient age ( P < .0002), whereas the plateau of thymic output was higher in patients receiving increased stem cell numbers ( P < .0022). Donor type, stem cell source, and conditioning regimen influenced none of the analyzed parameters. In conclusion, lymphopenia-driven proliferation is important for T-cell homeostasis in children early after stem cell transplantation, but it might result in underestimation of thymic function. Onset and plateau of thymic activity are independently regulated by different transplant-related factors.

Summary A possible link between protein kinase C (PKC) and P-glycoprotein (P-gp)-mediated-multidrug resistance (MDR) was assumed from studies on MDR cell lines selected in vitro. The functional relevance of PKC for the MDR phenotype remains unclear, and the involvement of a particular PKC isozyme in clinically occurring drug resistance is not known. Recently, we have demonstrated significant correlations between the expression levels of the PKC'q isozyme and the MDR1 or MRP (multidrug resistance-associated protein) genes in blasts from patients with acute myelogenous leukaemia (AML) and in ascites cell aspirates from ovarian cancer patients. To extend these findings to further types of human tumours we analysed specimens from 64 patients with primary breast cancer for their individual expression levels of several MDR-associated genes (MDR1, MRP, LRP (lung cancer resistance-related protein), topoisomerase (Topo) Ila/lip, cyclin A and the PKC isozyme genes (a, P,, P21 ii, 0 and j) by a cDNA-PCR approach. We found significantly enhanced mean values for MRP, LRP and PKCn gene expression, but significantly decreased Topo Ila and cyclin A gene expression levels in G2 tumours compared with G3.Remarkably, significant positive correlations between the MDR1, MRP or LRP gene expression levels and PKCr were determined: MDR1/PKC0 (rs = +0.6451, P < 0.0001) n = 62; MRPIPKCri (r, = +0.5454, P < 0.0001) n = 63; LRPIPKGYj (r, = +0.5436, P < 0.0001) n = 62; MRPILRP (rr = +0.7703, P < 0.0001) and n = 62, MDR1/MRP (rs = +0.5042, P< 0.0001) n = 62. Our findings point to the occurrence of a multifactorial MDR in the clinics and to PKCn as a possible key regulatory factor for up-regulation of a series of MDR-associated genes in different types of tumours.Keywords: Breast cancer; MDR; MDR1; MRP; LRP; PKC; topoisomerase 11The successful treatment of breast cancer using antineoplastic agents is often limited by the occurrence of drug resistance. Studies on cell lines selected in vitro revealed different mechanisms eventually responsible for the observed multidrug resistance (MDR), such as the overexpression of (a) the P-glycoprotein (P-gp) drug transporter (for review see Germann, 1996), (b) the 'multidrug resistance-associated protein' (MRP) that represents another ATP-binding cassette membrane glycoprotein also transporting drug conjugates (for review see Loe et al, 1996), (c) the 'lung cancer resistance-related protein' (LRP) that was described first in a MDR cell line lacking enhanced P-gp expression, characterized as a human major vault protein (for review see Izquierdo et al, 1996) and (4) a decreased activity of topoisomerase II (Topo II) (for review see Nitiss and Beck, 1996).An influence of protein kinase C (PKC) on P-gp-mediated MDR is suggested as well, but it still remains unclear how PKC or particular PKC isozymes might be functionally involved here: (a) in vitro studies on cell lines gave evidence that P-gp is directly

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The major determinants mediating drug resistance in acute lymphoblastic leukemias (ALL) unresponsive to chemotherapy, are still unclear. For example, it is still unknown whether selection or induction processes are responsible for drug resistance here or whether protein kinase C (PKC) isozymes contribute to the resistant phenotype. Therefore, inducibility of resistance factors or PKC isozymes genes was examined in CCRF-CEM cells treated with diverse anticancer drugsadriamycin, camptothecin, etoposide or vincristine-at sublethal concentrations for 24 h. MDR1, MRP1, LRP and PKC isozyme α α α α, β β β