Implications of early apoptosis of infected cells as an important host defense

Evaluating the Remote Control of Programmed Cell Death, with or without a Compensatory Cell Proliferation

Organisms and their different component levels, whether organelle, cellular or other, come by birth and go by death, and the deaths are often balanced by new births. Evolution on the one hand has built demise program(s) in cells of organisms but on the other hand has established external controls on the program(s). For instance, evolution has established death program(s) in animal cells so that the cells can, when it is needed, commit apoptosis or senescent death (SD) in physiological situations and stress-induced cell death (SICD) in pathological situations. However, these programmed cell deaths are not predominantly regulated by the cells that do the dying but, instead, are controlled externally and remotely by the cells' superior(s), i.e. their host tissue or organ or even the animal's body. Currently, it is still unclear whether a cell has only one death program or has several programs respectively controlling SD, apoptosis and SICD. In animals, apoptosis exterminates, in a physiological manner, healthy but no-longer needed cells to avoid cell redundancy, whereas suicidal SD and SICD, like homicidal necrosis, terminate ill but useful cells, which may be followed by regeneration of the live cells and by scar formation to heal the damaged organ or tissue. Therefore, “who dies” clearly differentiates apoptosis from SD, SICD and necrosis. In animals, apoptosis can occur only in those cell types that retain a lifelong ability of proliferation and never occurs in those cell types that can no longer replicate in adulthood. In cancer cells, SICD is strengthened, apoptosis is dramatically weakened while SD has been lost. Most published studies professed to be about apoptosis are actually about SICD, which has four basic and well-articulated pathways involving caspases or involving pathological alterations in the mitochondria, endoplasmic reticula, or lysosomes.

Replication or death: distinct fates of pathogenic Leptospira strain Lai within macrophages of human or mouse origin

Pathogenic leptospires evoke severe diseases in humans but only cause mild chronic or asymptomatic infection in many host animals. The reasons for this diversity of infection remain unclear. Here, we demonstrated that Leptospira interrogans serovar Lai strain Lai had a similar ability to adhere to and enter primary and immortal (THP-1 and J774A.1) macrophages from human and mouse, but its intracellular fate in human macrophages differed markedly from that in mouse. The leptospires resided within membrane-bound vacuoles in the murine macrophages, but occurred free in the cytosol of human macrophages, with no surrounding vesicular membrane. Most leptospires in murine macrophages co-localized with the late-endosomal/lysosomal marker LAMP-1 and then were killed by lysosomal hydrolases, while most leptospires in human macrophages did not co-localize with this marker and survived. Enumeration of colony-forming units plus quantitative fluorimetry showed that in human, but not in murine, macrophages, the amounts of leptospires increased with incubation time. The infected human macrophages differed from mouse macrophages by displaying gradually enhanced apoptosis, in parallel with the increase in number of leptospires. These data strongly suggest that the outcome for intracellular leptospires depends on differences among host macrophages, which may account for some of the differences in the severity of leptospirosis in humans and animals.

How the immune and nervous systems interact during disease-associated anorexia

Anorexia is one of the most common symptoms associated with illness and constitutes an adaptive strategy in fighting acute infectious diseases. However, prolonged reduction in food intake and an increase in metabolic rate, as seen in the anorexia-cachexia syndrome, lead to depletion of body fat and protein reserves, thus worsening the organism's condition. Because the central nervous system controls many aspects of food intake, soluble factors known as cytokines that are secreted by immune cells might act on the brain to induce anorexia during disease. This review focuses on the communication pathways from the immune system to the brain that might mediate anorexia during disease. The vagus nerve is a rapid route of communication from the immune system to the brain, as subdiaphragmatic vagotomy attenuates the decrease in food-motivated behavior and c-Fos expression in the central nervous system in response to peripheral administration of the proinflammatory cytokine, interleukin-1beta, or bacterial lipopolysaccharide. At later time points after peripheral lipopolysaccharide administration, interleukin-1 itself acts in the brain to mediate anorexia and is found in the arcuate nucleus of the hypothalamus. The mechanisms by which interleukin-1beta gains access to the brain and the potential role of neuropeptide-Y-containing neurons in the arcuate hypothalamus in mediating anorexia during disease are discussed.

Why infection-induced anorexia? The case for enhanced apoptosis of infected cells

A medically important paradox is why the body's own cytokines lead to reduced appetite and apparently inefficient metabolism as part of the acute-phase response. This self-induced nutrient restriction occurs just when the body must maintain a fever and other defensive functions. This paradox is often ignored or considered a metabolic derangement. Others, recognizing it to be a programmed response which must have net beneficial effects, consider the nutrient restriction to be an attempt to deny resources to infectious organisms. However, this explanation fails to address how the pathogen can be harmed more than the host. The hypothesis presented here offers an explanation. Apoptosis, or cell suicide, is becoming recognized as a useful defense against intracellular parasites, and nutrient restriction promotes apoptosis. Thus, nutrient restriction may encourage apoptosis of infected cells. Nutrient restriction can thereby offer protection by simultaneously limiting nutrients to both the host cells and the infectious organisms.