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Laser stimulated desorption (LSD) of neutral particles has been investigated in compound semiconductors MX of GaN, GaP, GaAs and CdS. No effects of crystallographic orientations on the characteristics of LSD, desorption yields, mean kinetic energies, and speed ratios have been recognized among (111) and (100) orientations in GaP and GaAs. This fact suggests that LSD does not follow the unimolecular dissociation of locally excited surface bonds, but occurs via a bulky event involving a phase transtion. The threshold laser powers for LSD were correlated to the crystal bond ionicities f i, which are good measures for the structural stability of tetrahedral covalent solids. It was found that with increasing f i of the solids, threshold laser powers tend to decrease. The dominant species ejected from MX were found to be in the forms of monomer M and X as well as dimer X2. The ratio of the yields of X2 to those of X increases as the single-bond energy of X2 increases. Considering these experimental results, we propose a model of a non-local electronic excitation mechanism for LSD; namely, LSD occurs via the dimerization enhanced phase transition in a high-density electron-hole plasma.

Abstract. Chemokines are known to have anti-tumor effects due to their chemoattractant properties, which stimulate the accumulation of infiltrating immune cells in tumors. CCL22 (macrophage-derived chemokine, MDC) attracts killer T cells, helper T cells and antigen-presenting cells expressing the CCL22 receptor, CCR4. Thus, CCL22 gene expression results in the accumulation of these cells in tumors, and has been shown to suppress lung and colon cancer growth in mice. In the present study, early-stage subcutaneous tumor growth in a mouse multiple myeloma cell line stably expressing CCL22 (MPC-CCL22) was decreased compared to tumor growth in control cells (MPC-mock). However, the final extent of tumor growth in these cell lines was almost equivalent. Regulatory T cells, which express CD25, CD4 and CCR4, are known to cause immune disruption. We therefore investigated the association of regulatory T cells with the progressive decrease in CCL22 anti-tumor effect observed in late-stage experimental multiple myelomas. Tumor growth in MPC-CCL22 cells was observed to drastically decrease, to the point of complete tumor regression, when CD4 or CD25 T cells were depleted. Here, we document the drastic anti-tumor effect of a combination of CCL22 and anti-CD25 antibody on multiple myeloma cells. IntroductionChemokines are a family of small cytokines that primarily induce the directed migration of hematopoietic cells when bound to their seven-transmembrane, G protein-coupled receptors (1,2). Because they function as chemoattractants for several immune effector cell types, chemokines are attractive candidates for immune cell-based approaches to cancer gene therapy. Several studies describe the anti-tumor effects induced by infiltrating immune cells upon their chemokine-mediated accumulation within the tumor mass. The regression of XCL1 (lymphotactin)-expressing engineered myeloma was mediated by increased infiltration of T cells, neutrophils and macrophages, leading to a reduction in tumorigenicity (3,4). CCL22 (macrophage-derived chemokine, MDC) is a CC chemokine that appears to be specifically secreted by a macrophage. CCL22 is a potent chemoattractant for CD8 and CD4 T cells, as well as for dendritic cells expressing the CCL22 receptor CCR4 (5,6), and is involved in chronic inflammation mediated by the continuous homing of dendritic cells and lymphocytes (7,8). Solid tumor growth in mice was suppressed by the recruitment of these cell types into tumors induced to express CCL22 using an adenoviral vector (9). Due to its tendency to function as a chemoattractant for killer T cells, helper T cells and antigen-presenting cells, CCL22 is considered to be an attractive candidate gene for immune cell-based cancer therapy. However, one drawback for such CCL22-based therapy is the immunosuppressive condition caused by the fact that CCL22 also recruits regulatory T cells into the tumor mass. Here, we demonstrate that a drastic anti-tumor CCL22 effect can be sustained in multiple myeloma tumors by reducing the recruitment of regulatory...

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