Infectious entry of enveloped viruses is thought to proceed by one of two mechanisms. pH-dependent viruses enter the cells by receptor-mediated endocytosis and are inhibited by transient treatment with agents that prevent acidification of vesicles in the endocytic pathway, while pH-independent viruses are not inhibited by such agents and are thought to enter the cell by direct fusion with the plasma membrane. Nearly all retroviruses, including amphotropic murine leukemia virus (MuLV) and human immunodeficiency virus type 1, are classified as pH independent. However, ecotropic MuLV is considered to be a pH-dependent virus. We have examined the infectious entry of ecotropic and amphotropic MuLVs and found that they were equally inhibited by NH 4 Cl and bafilomycin A. These agents inhibited both viruses only partially over the course of the experiments. Agents that block the acidification of endocytic vesicles also arrest vesicular trafficking. Thus, partial inhibition of the MuLVs could be the result of virus inactivation during arrest in this pathway. In support of this contention, we found that that the loss of infectivity of the MuLVs during treatment of target cells with the drugs closely corresponded to the loss of activity due to spontaneous inactivation at 37°C in the same period of time. Furthermore, the drugs had no effect on the efficiency of infection under conditions in which the duration of infection was held to a very short period to minimize the effects of spontaneous inactivation. These results indicate that the infectious processes of both ecotropic and amphotropic MuLVs were arrested rather than aborted by transient treatment of the cells with the drugs. We also found that infectious viruses were efficiently internalized during treatment. This indicated that the arrest occurred in an intracellular compartment and that the infectious process of both the amphotropic and ecotropic MuLVs very likely involved endocytosis. An important aspect of this study pertains to the interpretation of experiments in which agents that block endocytic acidification inhibit infectivity. As we have found with the MuLVs, inhibition of infectivity may be secondary to the block of endocytic acidification. While this strongly suggests the involvement of an endocytic pathway, it does not necessarily indicate a requirement for an acidic compartment during the infectious process. Likewise, a lack of inhibition during transient treatment with the drugs would not preclude an endocytic pathway for viruses that are stable during the course of the treatment.

Intestinal epithelial cells secrete a protective luminal mucus barrier inhibiting viral gene transfer. Quiescent, polarized monolayers of primary epithelial cells from dog gallbladder and human colon are efficiently transduced through the apical mucus side by lentivirus vectors, suggesting their application to intestinal gene therapy.

Cyclic neutropenia is a rare disease that occurs both in humans and gray collie dogs and is characterized by recurrent severe neutropenia leading to bacterial infections and shortened life expectancy. Daily injections of recombinant granulocyte colony-stimulating factor (rG-CSF) are effective in shortening the period of severe neutropenia and reducing infections. After demonstrating that rG-CSF induced elevated neutrophil production in an affected dog, cytokine administration was stopped and 10 9 infectious units (IUs) of a lentivirus pseudotyped with vesicular stomatitis virus G protein (VSV-G) encoding canine G-CSF cDNA was administered intramuscularly. Serial blood cell counts showed elevated neutrophil production for longer than 17 months. Although neutrophil counts continued to cycle, the range at nadirs was from 3710 to 5300 cells/L, well above the nadirs before lentivirus administration. After the injection of lentivirus, mean neutrophil counts ؎ SD were 12 460 ؎ 4240 cells/ L, significantly increased over both pretreatment values of 3040 ؎ 2540 cells/L (P < .0001) and neutrophil counts during G-CSF administration of 10 290 ؎ 4860 cells/L (P < .007). The changes in blood counts from lentivirus injection were associated with absence of clinical signs of infection and fever. The gray collie continued to gain weight and was no longer housed in a pathogen-free environment. Genomic DNA from muscle at injection sites was positive for provirus, whereas gonad, lung, spleen, heart, liver, kidney, leukocytes, and noninjected muscle samples were all negative for provirus. Thus, intramuscular administration of lentivirus encoding G-CSF provided sustained therapeutic levels of neutrophils, suggesting this approach may be applied for long-term treatment of patients with cyclic and other neutropenias. IntroductionCyclic neutropenia is a rare disease that occurs both in man and gray collie dogs. In collie dogs, cyclic hematopoiesis is inherited as an autosomal recessive disease. 1,2 The dogs have 12-to 14-day cyclic variations in blood neutrophils, monocytes, lymphocytes, eosinophils, reticulocytes, and platelets due to periodic fluctuations in blood cell production by the bone marrow. 1,2 The recurrent severe neutropenia leads to bacterial infections and shortened life expectancy. The disorder can be cured by bone marrow transplantation from a normal dog to a gray collie as well as transferred from a gray collie to a normal littermate. 3,4 This transplantability strongly supports the concept that this is a disease of defective regulation of hematopoietic stem cells.Gray collie dogs with cyclic hematopoiesis have been successfully treated with granulocyte colony-stimulating factor (G-CSF). 5,6 The severe recurrent neutropenia in gray collie dogs was not abrogated by in vivo interleukin 3 (IL-3) or granulocytemacrophage colony-stimulating factor (GM-CSF) treatment. 5 In contrast, G-CSF prevented the recurrent neutropenia and obliterated periodic fluctuation of monocyte, eosinophil, reticulocyte, and platelet counts. 5,6 ...

Human cyclic neutropenia, also referred to as cyclic hematopoiesis, is a rare disease characterized by periodic fluctuations in blood cell production by the bone marrow and a corresponding recurrent severe neutropenia every 19-21 days. This results in bacterial infections and shortened life expectancy. Platelets, monocytes, lymphocytes, and reticulocytes cycle with the same periodicity. It has been determined that the neutrophil elastase (NE) gene is mutated in all cases of human cyclic hematopoiesis. Currently, the only animal model for this disease is the grey collie dog, in which there is a strikingly similar periodic neutropenia every 12-14 days. Towards the validation of this animal model, we have cloned and sequenced the canine NE cDNA from a normal dog.