Granulocytes, or polymorphonuclear leukocytes, form the first line of defense in the human body against invading pathogens. They are by far the most abundant white blood cells in the body, constituting 50-70% of all circulating white blood cells. There are three types of granulocytes - neutrophils, eosinophils and basophils - with neutrophils comprising more than 90% of granulocytes. As a result, much of the research on granulocytes has focused on neutrophils and in many instances, as in the present case, the two names are used interchangeably.
Granulocytic differentiation (granulopoiesis) comprises six defined stages, each distinguished by specific molecular and morphological changes. Deriving from CFU-G progenitor cells, myeloblasts are the earliest detectable granulocytic precursors. Committed granulocytes evolve from myeloblasts into promyelocytes, myelocytes, metamyelocytes, bands and finally mature segmented granulocytes. Granulocyte maturation is marked by increased ex
Granulocytic maturation is also characterized by both a progressive decrease in cellular and nuclear size and an increase in cytoplasmic granules. Cellular size decreases from 20 to 25 microns in diameter for myeloblasts to 20 microns in diameter for promyelocytes to 12 to 18 microns in diameter for myelocytes, metamyelocytes and bands. Fully mature, segmented neutrophils are approximately 12 –14 microns in diameter. As neutrophils mature, their nuclei also undergo profound changes. Myeloblasts contain large, oval nuclei that begin to become concave as chromatins begin to condense in cells entering the promyelocyte and myelocyte stages. Chromatin condensation continues as the nuclei become kidney-bean-shaped at the metamyelocyte and band stages and finally segment into 2 to 5 lobes connected by thin chromatin filaments in fully mature, segmented cells. The segmentation of nuclei is believed to aid in neutrophils’ flexibility and thus, their extravasation into tissues and through tight spaces. Cytoplasmic granules, containing harmful proteins that can destroy invading organisms, begin appearing at the myeloblast stage and rapidly increase in number as cells mature.
Granulocytes pass through three different regions of the body during their short lifespan, the bone marrow, blood, and tissues. Both granulocyte proliferation and differentiation occur in the bone marrow. The bone marrow is infused with a network of sinuses that oxygenate and nourish it. The sinuses all drain into a central vein. Hematopoietic activity occurs in the extravascular spaces between these sinuses, with granulocyte proliferation and early maturation occurring deep within the spaces where the O2 level is lower than areas adjacent to the sinuses. The exact O2 level at which granulopoiesis occurs is unknown, but the average O2 level within the bone marrow is approximately 7%. Accordingly, ex vivo granulocytes cultured under a low pO2 of 5% exhibit 2 to 3 fold higher expansions than those cultured under standard culturing conditions of 20% O2. At the metamyelocyte stage, granulocytes begin to migrate towards the sinus wall as they continue to mature. Fully mature, segmented neutrophils cross the sinus wall into the circulating blood.
Peripheral blood contains two granulocyte populations in equilibrium, the circulating pool and the marginal pool. The number of cells in each pool is approximately the same. On average, cells in the circulating pool live 6- 10 hours and survey the body for invading pathogens as they travel through the blood. Cells in the marginal pool remain adhered to the walls of blood vessels for an unknown amount of time. Upon pathogen infection or other stress, cells in the marginal pool are released into the circulating pool and are often recruited to sites of infection.
Circulating granulocytes exit the bloodstream and enter inflamed tissues through the process of extravasation. Extravasation is initiated by a sequence of tethering steps, when a circulating granulocyte forms weak contacts with the endothelial cells that line the walls of blood vessels. The force of the blood flow, however, breaks the contacts. Thus, the granulocyte is slowed down but not stopped and it continues to roll along the wall of the blood vessel. Chemokines released by the tissues mediate stronger adhesion between endothelial and granulocytic cells that finally stops, or traps, the granulocytic cells. The tight adhesions mediated by the ICAMs and activated LFA-1 spreads the granulocytic cell and leads to migration of the cell between adjacent endothelial cells into the surrounding tissue. Once inside the tissue, granulocytes follow a gradient of inflammatory chemokines to the site of infection. At sites of infection, granulocytes utilize three main functions to kill invading organisms, phagocytosis, exocytosis of cytotoxic proteins and production of reactive oxygen species through a process of respiratory burst.
Though the morphological and functional properties of granulocytes are well characterized, only a small number of genes that regulate or are involved in each process are known. Hematopoietic cells, including granulocytes, are most commonly identified by their pattern of surface marker ex
Perhaps the largest category of known genes in granulocytes codes for the granule proteins. Upon stimulation, granule proteins are released into the extracellular surroundings or phagosomes (formed through the process of phagocytosis) to help combat foreign organisms. Approximately 30 – 40 proteins have been identified belonging to that group, with defensin (DEF), myeloperoxidase (MPO), lactotransferrin (LTF), and elastase (ELA2) being the most well-known. Despite the large number already known, new granule proteins continue to be identified with haptoglobin being the newest identified member. Additionally, the processes by which the proteins are sorted into different granules and released remain unclear.