stephieku: (lyra)
[personal profile] stephieku
Hey everyone! I haven't touched this blog in forever. I'm terrible at blogging personal entries, so I'm going to use this journal as a way to force me into reading more scientific papers. The goal is to write a summary on a scientific research paper daily! Since I'm doing this for myself, feel free to ignore these entries, but go ahead and comment if it interests you, or if you have questions!

Most of these papers will be related to my research, so I apologize if they're repetitive or boring.

Title: An analysis of the structural and functional similarities of insect hemocytes and mammalian phagocytes
Authors: Niall Browne, Michelle Heelan, and Kevin Kavanagh

Despite insects diverging from vertebrates about 500 million years ago, vertebrates and insects have structurally and functionally similar innate immune systems. Insects don't have an adaptive immune response, only an innate immune response. The insect immune response is made of two components: the cellular and humoral reponses. The cellular response is mediated by hemocytes. Hemocytes functions include phagocytosis, encapsulation, and clotting. The humoral response is made of soluble effector molecules such as anti-microbial peptides, complement-like proteins, melanin and other killing products. These are often produced in the insect's fat body.

There are at least eight types of hemocytes, but most insects do not have all types. Hemocytes circulate in the hemolymph or attach to internal organs like the fat body or digestive track. The density of hemocytes in the hemolymph is dependant on the life stage of the insect and pathogen response. Similar to vertebrates, hemocytes can recognize foreign materials and distinguish self from non-self. Studies have compared the virulence of bacteria and fungus in model insects and model vertebrates.

Types of Hemocytes
Prohemocytes: small circular cells with a large nucleus. Its basophilic cytoplasm can differentiate into different types of cells.
Plasmotocytes: most common hemocyte. Leaf shaped and contain lysomal enzymes. Involved in capsule formation
Granular cells: small nucleus and many granules in the cytoplasm.
Spherulocytes: Many different shapes
Oenocytoids: large, double nucleus, non-phagocytic
Coagulocytes: involved in clotting

Like in mammals, phagocytosis is enabled by opsonization (coating of particles to mark for phagocytosis) of microorganisms with complement-like proteins.In order to initiate phagocytosis, the pathogen must be recognized as foreign and initiate signalling pathways. Once recognized, ligands bind to the outside of the pathogen. In phagocytosis, the pathogen is engulfed and superoxide production kills the pathogen. However, this also results in damage in nearby tissue and causes an inflammatory response. Lectins and lysozymes act simultaneously to break down the cell layers.

Nodulation is when many hemocytes bind to clusters of bacteria. Hemocytes bind toegther and wrap around the pathogen. This is the main cellular defense reaction in insects and it allows a lot of bacteria to be removed at once. There is no equivalent function in humans.

For larger pathogens. There are two different types which occur in different orders: cellular and humoral. Humoral encapsulation can occur with hemocytes, while celullar encapsulation occues without melanization. A foreign object is recognized by granular cells which release materials that cause plasmatocytes to attach. Multiple layers of plasmocytes attach and create a capsule. In cellular encapsulation, lamellocytes were more frequently used than plasmatocytes and granular cells.

Vertebrates phagocytes

Vertebrate and insect innate immune systems are similar. Phagocytes function similarly to plasmatocytes and granular cells. When a phagocyte engulfs an object, it extends pseudopods around it until it's engulfed. It is contained in an endosome (membrane bound compartment), which is merged with a lysosome that contains enzumes and acids that digest the object.

Phagocytic cells
Macrophages: large phagocytic leukocytes that can cross the cell membrane of capillary vessels. Very effecient and can consume a large number of pathogens. When pathogens bind to the macrophage, the macrophage engulfs it and activates a respiratory burst (release of reactive oxygen species).It also causes the macrophage to release chemokines that attracts other immune cells to the site of infection.
Neutrophils: Mot common phagocyte and usually first to arrive at infection site. Have numerous granules in their cytoplasm (granular cells). They are similar to insect granular cells and plasmatocytes. Production of neutrophils increases during infection. Like insect hemocytes, they use respiratory burst to attack pathogens.
Dendritic cells: Present antigen by exogenous and endogenous routes to naive cells. They capture and process antigens, then initiate specific immune responses in naive T and B cells in the lymph node. DCs can detect microbial components in the environment and enable activation of the adaptive immune response. (there is a bit more, but I'm sleep deprived)

Insect Hemocytes and Mammalian Phagocytes: Similarities and Differences
- Functional similaries with similar effectors and receptors
- Homology in immune proteins
- Receptors on phagocytic cells in insects are similar to receptors on mammalian neutrophils
- Phagocytic cells in both engulf and kill pathogens using superoxide
- Neutrophils and granular cells respond to inhibitors in similar ways
- Some pathways are similar, but used for different purposes (i.e. Toll/Dorsal signalling pathway)
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