Week X and Final Goals

I apologize for the lateness of this final summery; tying up the scope of my research has been tricky and required a long period of meditation.

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Week IX

This week has focused on cytokine production and specialized activation of T cell subtypes included in my model (Th17, Th1, Th2) as well as microglia and CNS invasive CD11+ expressing cells like macrophages and dendritic cells. There are many involved and I won’t go into the specifics of what produces what and where they go, only that several pool into the same compartment and add to one amount. Main cytokine circulation occurs in the CNS and lymph node (LN). PGE2 serves a prominent loe in the activation and feedback regulation of IL-23 and IL-17 for TH1 and Th2 activity. I have T cells being activated and interacting with B cells and naturally in-circulating DCs and macrophages. PrPSc enters the LN after being taken up in an unclear method by M cells from the intestinal lumen like other pathogen, and then cross infect cells in Peyer’e Patches that regular populate them and then go into draining LNs. Either by uptake of infection, DCs and macrophages become prion carriers, once in the lymph node they can interact with B cells or directly within germinal centers (which may be preformed by a pre-existing infection). Follicular dendritic cells which highly express PrPC become proliferation sites for the scrapie protein, cross infecting T cells, which then recirculate into the blood and to new sites of injury with a pro-inflammatory goal. If T cells are recruited into the CNS in response to damaged neurons they will cross the BBB barrier, otherwise it seems unlikely that the T cells will enter the CNS enough if at all to infect cells there. Therefore, in pre-existing CNS damage is not occurring, how does the prion enter the brain to trigger more damage and more frequent neuroinvasion of cytotoxic and regulatory cells all potentially cross-infected with prions. This has yet to be elucidated.

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This week has been a difficult, but has produced a new positive direction for the proliferation components of my model. Early in my research I came across antigen presentation to immune cells as a quintessential component of inflammation. DCs and macrophages can act as APCs (antigen presenting cells) and as well as being able to carry PrPSc in their regular circulation. I dismissed the subject because I found no evidence of PrPC or PrPSc acting as anitgens or that antibodies were formed for their ligation. I saw no dire reason to include dynamic interactions with T cell (and several subtypes) if there was nothing to engage their specific immune response. I was wrong. Though DCs through induced maturation process do migrated between the blood, brain, and lymphodial system, they do not directly interact with the non-migratory FDCs in the spleen and lymph node B cell follicles/germinal systems. I have pinpointed FDCs as key spots for PrPSc accumulation and conversion after peripheral infection, but it remained unclear how they left these areas except that neuroinvasion was seem after a certain critical titer load of the scrapie protein. So I looked at what comes in contact with FDCs, turns out B cells and naïve T cells circulate into these areas, and likes a game of tragic telephone can transfer the scrapie protein between them. T cell response can be induced by a prion infection, become primed in the LN or spleen and then reactivated in the perivascular space following cytokine and growth factor mediated TEM of the CSF and BBB. Macrophages and phagocytic DC act as APCs for the reactivation of migrated T cells and then relocalized to neurons damaged by a previous viral infection. A previous infection may not be necessary for prion proliferation because inflammation could be an indirect result of PrP loss-of-function, but an induced specific immune response in concatenation with peripheral prion infection could significantly increase the severity to the disease.

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Week VII

The focus this week has been on monocyte, DC, and T-cell trans endothelial/epithelial migration during inflammation. Though this phenomenon is not unique to prion disease, it is an important aspect to my model and connects it to other neurodegenerative diseases, like AD. ICAM-1 expression and subsequent ligation by a variety of proteins, like adherins expressed on the outer membrane of localized macrophages, attracted to sites of inflammation by afore mentioned inflammatory cytokines (IL-1β and TNFα), is important for transmigration of these gathered cells into the CNS and lymphatic system. ICAM-1 can also be the initial site for multiple signal transduction pathways including MAPK, ERK1/2, JNK, and activation of NF-κB. These pathways result in upregulation of  ICAM/VCAM  expression, cytokine release, Ca2+ efflux, and disruption of ECM. The disruption of the ECM is vital for the clustering component of ICAM-1, along with other proteins that aid monocyte/macrophage rolling and transmigration. The blood brain barrier is composed of endothelial cells that maintain a highly selective barrier between the blood and CNS with tight junctions (which include PECAM co-localized with endothelial-expressed PrPC, the implications of which have been previously discussed).  Cell can cross the barrier via two mediated paths: trans- and –paracellular migration. Paracellular migrations requires disruption of the tight leading to greater perforation of the BBB by migratory inflammatory cells, and greater damage in the CNS. One study claimed that though the method of choosing para vs. trans migration remains unclear, both have been witnessed during inflammation. It may be the T-cells prefers a paracellular route and monocytes prefer the trans. The internalization of ICAM-1 into caveosomes has been implemented in the transport of leukocytes into the endothelial cell, into MVBS, and released into the CNS in exosomes. The rate of each migration path need be elucidated; it may have a significant on the overall rate of disease pathogenesis as disruption of the BBB by continuous paracellular migration of leukocytes can increase the number of cells entering the CNS.

ICAM-1 Signaling and Role in Leukocyte Migration

ICAM-1 (and not so abundantly expressed VCAM-1) are essential components in leukocyte migration across or even through edothelial cells. Endothelial cells make up the barrier between blood vessels and neighboring system, like the brain. The blood brain barrier is actually composed of a highly selective layer of endothelial cells that are interwoven with both tight junctions and adherins. Transendothelial migration or TEM, can be induced by pro-inflammatory factors such as TNF[alpha]. It can activate a number of signaling pathways, including one culmination in NF-kB activation. TNF[alpha] can upregulate the expression of strong adhesion ICAM-1 on the surface of endothelial cells. They cluster on the outer cell membrane and attach to expressed adhesion molecule, LFA-1, on circulating leukocytes. This recruits leukocytes to endothelial cells and induces rolling. TEM can occur either transcellularly or paracellularly. In the case of crossing the BBB, a transcellular pathway is preferential, as it does not require the disruption of tight junctions. It is unclear whether ICAM-1 is drawn into internal compartments or special organelles called LBRCs with the aid of caveolin-1 and formation of caveolae and onto MVBs. The later seems more plausible, but may also involve the internalization of of VE-cadherins. Leukocyte binding or other ligation to ICAM-1 may trigger a number of responses including activation of c-Fos and c-Jun transcription factors to upregulate ICAM-1. It may also cause the build up of ROS through NADPH oxidase activation, and thereby activating NF-[kappa]B and increase production of ICAM-1 and cytokines. ROS also contributes to cytoskeletal changes, which is necessary for the clustering of ICAM-1.

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