Circulating effector T cells preferentially home to peripheral tissue sites of infection rather than lymphoid organs because of changes in adhesion molecule and chemokine receptor expression. Many of the protective antimicrobial functions of effector T cells must be performed locally at sites of infections, and, therefore, these cells must be able to leave lymphoid organs and migrate into infected tissues. After differentiation of naive T lymphocytes into effector cells, when S1PR1 expression is restored and expression of CCR7 and L-selectin is lost, the effector T cells also start expressing receptors for inflammatory chemokines produced at infection sites and the ligands for E- and P-selectins. These changes impair homing of the effector T cells into other secondary lymphoid organs and promote their migration into sites of infection.

The process of effector lymphocyte homing into infected tissues occurs in postcapillary venules and is mediated by the same multistep selectin-, integrin-, and chemokine-dependent process described for other leukocytes. As with neutrophils and monocytes, effector T cells in the circulation, but not naive T cells, express selectin ligands, integrins, and chemokine receptors that bind to the types of selectins, integrin ligands, and chemokines, respectively, that are expressed in activated endothelium (see Fig. 1).

Fig1. Molecules involved in migration of naive and effector T lymphocytes. (A) Naive T lymphocytes home to lymph nodes as a result of L-selectin binding to peripheral node addressin (PNAd) on high endothelial venules (HEVs), which are present only in secondary lymphoid organs, and as a result of binding chemokines (CCL19 and CCL21) displayed on the surface of the HEV by glycosaminoglycans. Activated T lymphocytes, including effector cells, home to sites of infection in peripheral tissues, and this migration is mediated by E-selectin and P-selectin, integrins, and chemokines that are produced at sites of infection. Additional chemo kines and chemokine receptors, besides the ones shown, are involved in effector/memory T-cell migration. (B) The adhesion molecules, chemokines, and chemokine receptors involved in naive and effector/memory T-cell migration are described. ICAM-1, Intercellular adhesion molecule 1; LFA-1, leukocyte function–associated antigen 1; VCAM-1, vascular cell adhesion molecule 1; VLA-4, very late antigen 4.

The migration of effector T cells into infected tissues is antigen-independent, but the effector cells that recognize an antigen in the tissue are preferentially retained there. Thus, effector cells of diverse specificities can enter tissue sites of infection, which maximizes the chance of cells finding the anti gen for which they are specific. If some of the T cells recognize an antigen in infected tissues, their integrins are kept in the high-affinity state because of antigen-induced activation. These integrins bind tightly to extracellular matrix proteins, and this favors the retention of effector T cells that encounter antigens at these sites. Retention allows effector T cells that recognize anti gens to perform the functions that eliminate microbes and other sources of the antigens. Most effector cells that enter a site of infection eventually die there after performing their functions.

Some effector cells have a propensity to migrate to particular types of tissues. This selective migration capacity is acquired during the differentiation of the effector T cells from naive pre cursors in secondary lymphoid organs. By enabling distinct groups of effector T cells to migrate to different sites, the adaptive immune system directs cells with specialized effector functions to the locations where they are best suited to deal with particular types of infections. The clearest examples of populations of effector T cells that specifically home to different tis sues are skin-homing and gut-homing T cells, whose migration patterns reflect the expression of different adhesion molecules and chemokine receptors on each subset, discussed in detail in Chapter 14.

Different subsets of effector T cells exist, each with distinct functions, and these subsets have different although oftenover-lapping patterns of migration. Effector T cells include CD8+ cytotoxic T lymphocytes (CTLs) and CD4+ helper T cells (Th). Helper T cells include Th1, Th2, Th17, and T follicular helper cells (Tfh) subsets, each of which expresses different types of cytokines and preform different functions. After their differentiation in lymphoid tissues, CTLs and Th cells mainly migrate out to the sites of infection. The characteristics and functions of these subsets will be discussed in detail in Chapter 10. For now, it is sufficient to know that the migration of these subsets shows some differences. This is because the array of chemokine receptors and adhesion molecules expressed by each subset differs in ways that result in preferential recruitment of each subset into sites of inflammation elicited by different types of infections. On the other hand, Tfh, whose function is to help B cells, express chemokine receptors shared by B cells, and therefore stay in the lymphoid organs and migrate toward B-cell follicles.

اخر الاخبار

اخبار العتبة العباسية المقدسة

العتبة العباسية المقدسة تقيم مجلس عزاء لإحياء ذكرى شهادة الإمام جعفر الصادق (عليه السلام)

بعد مرور أربع سنوات على تأسيسه قسم شؤون المعارف يعلن منجزات مركز التراث الإسلامي في إيران

قسم الشؤون الفكرية يختتم مسابقة (معز المؤمنين الثقافية) ويكرِّم الفائزين في كربلاء وبابل

قسم الصيانة ينفذ أعمالًا تطويرية في مجمع الإمام الهادي (عليه السلام)

المزيد

الآخبار الصحية

الشاشات ليست مجرد ترفيه.. تأثيرات طويلة المدى على دماغ طفلك

الأسباب الرئيسية لنقص الطاقة في الجسم

الكاكاو وتأثيره على العين.. نتائج واعدة من تجارب مخبرية

طبيب يحذر: أطعمة شائعة الاستهلاك أخطر على صحتنا من التدخين

المزيد

الآخبار التكنلوجية

باحثون في "نيويورك أبوظبي" يطورون تقنيات لرصد وعلاج الأورام

جيتكس إفريقيا.. المغرب يجمع "رواد التكنولوجيا" من 103 دول

طاقم أرتميس 2 يحطم رقما قياسيا ويستعرض الجانب البعيد للقمر

"أرتيميس 2" تنطلق حول القمر وتحطم الرقم القياسي

المزيد

مواضيع ذات صلة

Rh Blood Types

O-A-B Blood Types

Allergy and Hypersensitivity

The Antiviral Response

Migration of B Lymphocytes

Exit of Naive and Effector T Cells from Lymph Nodes

Lymphocytes are Responsible for Acquired Immunity

The Major Proinflammatory Cytokines of Innate Immunity

Migration of Naive T Cells Into Lymph Nodes

Migration of Neutrophils and Monocytes to Sites of Infection or Tissue Injury

Leukocyte-Endothelial Interactions and Leukocyte Recruitment into Tissues

Formation of Pus