Superoxide production by phagocytic leukocytes: the scientific legacy of Bernard Babior

The phagocyte respiratory burst: Historical perspectives and recent advances

When exposed to certain stimuli, phagocytes (including neutrophils, macrophages and eosinophils) undergo marked changes in the way they handle oxygen. Firstly, their rate of oxygen uptake increases greatly. This is accompanied by (i) the production of large amounts of superoxide and hydrogen peroxide and (ii) the metabolism of large quantities of glucose through the hexose monophosphate shunt. We now know that the oxygen used is not for respiration but for the production of powerful microbiocidal agents downstream of the initial production of superoxide. Concomitantly, glucose is oxidised through the hexose monophosphate shunt to re-generate the NADPH that has been consumed through the reduction of molecular oxygen to generate superoxide. This phagocyte respiratory burst is generated by an NADPH oxidase multi-protein complex that has a catalytic core consisting of membrane-bound gp91phox (CYBB) and p22phox (CYBA) sub-units and cytosolic components p47phox (NCF1), p67phox (NCF2) and p40phox (NCF4). Finally, another cytosolic component, the small G-protein Rac (Rac2 in neutrophils and Rac1 in macrophages) is also required for full activation. The importance of the complex in host defence is underlined by chronic granulomatous disease, a severe life-limiting immunodeficiency caused by mutations in the genes encoding the individual subunits. In this review, I will discuss the experimental evidence that underlies our knowledge of the respiratory burst, outlining how elegant biochemical analysis, coupled with study of patients deficient in the various subunits has helped elucidate the function of this essential part of innate immunity. I will also discuss some exciting recent studies that shed new light on how the abundance of the various components is controlled. Finally, I will explore the emerging role of reactive oxygen species such as superoxide and hydrogen peroxide in the pathogenesis of major human diseases including auto-inflammatory diseases.

The role of reactive oxygen species and nitric oxide in programmed cell death associated with self-incompatibility

Successful sexual reproduction often relies on the ability of plants to recognize self- or genetically-related pollen and prevent pollen tube growth soon after germination in order to avoid self-fertilization. Angiosperms have developed different reproductive barriers, one of the most extended being self-incompatibility (SI). With SI, pistils are able to reject self or genetically-related pollen thus promoting genetic variability. There are basically two distinct systems of SI: gametophytic (GSI) and sporophytic (SSI) based on their different molecular and genetic control mechanisms. In both types of SI, programmed cell death (PCD) has been found to play an important role in the rejection of self-incompatible pollen. Although reactive oxygen species (ROS) were initially recognized as toxic metabolic products, in recent years, a new role for ROS has become apparent: the control and regulation of biological processes such as growth, development, response to biotic and abiotic environmental stimuli, and PCD. Together with ROS, nitric oxide (NO) has become recognized as a key regulator of PCD. PCD is an important mechanism for the controlled elimination of targeted cells in both animals and plants. The major focus of this review is to discuss how ROS and NO control male-female cross-talk during fertilization in order to trigger PCD in self-incompatible pollen, providing a highly effective way to prevent self-fertilization.