Deoxycytidine kinase (dCK) is a rate-limiting enzyme in deoxyribonucleoside salvage, a metabolic pathway that recycles products of DNA degradation. dCK phosphorylates and therefore activates nucleoside analog prodrugs frequently used in cancer, autoimmunity, and viral infections. In contrast to its well established therapeutic relevance, the biological function of dCK remains enigmatic. Highest levels of dCK expression are found in thymus and bone marrow, indicating a possible role in lymphopoiesis. To test this hypothesis we generated and analyzed dCK knockout (KO) mice. dCK inactivation selectively and profoundly affected T and B cell development. A 90-fold decrease in thymic cellularity was observed in the dCK KO mice relative to wild-type littermates. Lymphocyte numbers in the dCK KO mice were 5-to 13-fold below normal values. The severe impact of dCK inactivation on lymphopoiesis was unexpected given that nucleoside salvage has been thought to play a limited, "fine-tuning" role in regulating deoxyribonucleotide triphosphate pools produced by the de novo pathway. The dCK KO phenotype challenges this view and indicates that, in contrast to the great majority of other somatic cells, normal lymphocyte development critically requires the deoxyribonucleoside salvage pathway.

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In order to analyze the molecular mechanism of sexual cell fusion between cells of HM1 and NC4 (opposite mating type strains in Dictyostelium discoideum), monoclonal antibodies were raised against partially-purified gp 70, a fusion-related protein of HM1 cells. The antibodies were screened for activity to inhibit cell fusion and 9 hybridoma clones were obtained. One of the fusion-blocking monoclonal antibodies, mAblG7, was used for further analysis. It recognized nearly ten bands in an immunoblot of fusion competent HM1 cells, but no bands when HM1 membrane proteins had been deglycosylated. These results suggest the importance of carbohydrates in the cell fusion process. To confirm this possibility, effects of sugars or lectins on cell fusion were examined. Although inhibition by the sugars was incomplete, Con A, WGA, LCA, strongly inhibited cell fusion. Furthermore, tunicamycin inhibited the acquisition of fusion competence in HMI cells, indicating the importance of N-linked glycosylation of proteins in cell fusion. All above results suggest that N-linked carbohydrates on HM1 cell surface are involved in the sexual cell fusion of D. discoideum.