4 - Linfocitos T

Name: 4 - Linfocitos T
Uploaded: 2021-12-02T14:43:11.000Z
Duration: 1 h 52 min 55 s

Understanding T Lymphocytes and Their Development

Overview of T Lymphocyte Generation

The seminar focuses on T lymphocytes, covering their generation, activation requirements, immune response profiles, and memory.

The T cell receptor (TCR) is formed during ontogeny before antigen exposure through a process called somatic recombination.

Structure of the T Cell Receptor (TCR)

The TCR consists of alpha and beta chains; the variable region of the beta chain shows significant variability compared to the more conserved carboxy-terminal regions.

The variable portion of the beta chain is illustrated in three colors to represent its diverse generation through genetic rearrangement.

Genetic Rearrangement Process

In non-T lymphocyte cells and precursor T cells, genetic material exists in a germline configuration until somatic recombination occurs.

A specific fragment (B1 from J segments) undergoes rearrangement, leading to a new beta chain with a variable region after transcription and translation.

Formation of Alpha Chain

The alpha chain's variable region is generated by combining two fragments known as B&J segments found in all precursor cells.

Enzymes called recombinases recognize these genetic fragments, cut them, and other enzymes join them together to form functional receptors.

Mechanisms for Diversity Generation

Somatic recombination allows for an extensive variety of B cell receptors (BCR), contributing to antigenic diversity due to imprecise joining mechanisms.

This diversity arises not only from different configurations but also from errors introduced during the joining process that affect nucleotide sequences.

Ontogeny in Thymus

Once successfully formed, the open-chain structure expresses on the membrane alongside a surrogate alpha chain; this triggers cellular proliferation upon successful arrangement.

Each daughter cell inherits this arrangement while further diversifying through additional combinations with various alpha chains generated during thymic development.

Importance of Precise Genetic Rearrangement

The specificity of receptors comes from how well they can bind antigens; thus, variations in nucleotide sequences enhance recognition capabilities.

Even if identical fragments are chosen for rearrangement, imprecise joining leads to unique nucleotide sequences that contribute to receptor diversity.

Migration and Training within Thymus

Hematopoietic stem cells migrate from bone marrow to thymus where crucial steps in lymphocyte development occur within distinct thymic lobules.

T-Cell Ontogeny and Thymocyte Development

Thymocyte Environment and Initial Stages

Thymocytes encounter a unique environment in the thymic cortex, primarily composed of cortical thymic epithelial cells, differing from those found in the medullary region.

The development of T lymphocytes involves specific markers; thymocytes are categorized based on co-receptor expression (CD4 and CD8). Those lacking both are termed double-negative cells.

Progression to Positive Selection

During T-cell ontogeny, thymocytes transition from double-negative to double-positive states as they begin expressing co-receptors, ultimately maturing into single-positive T lymphocytes.

Initially, double-negative thymocytes proliferate rapidly within the cortical zone without expressing CD4 or CD8. This stage is crucial for defining their lineage.

Rearrangement of TCR Genes

The majority of circulating T lymphocytes undergo significant rearrangements in their beta chain genes (V(D)J recombination), which is essential for functional T-cell receptor (TCR) formation.

Successful rearrangement leads to transcription and translation of the beta chain, allowing it to pair with a surrogate alpha chain on the thymocyte surface.

Positive Selection Mechanism

Once both chains are expressed on the cell membrane, signals trigger proliferation and further maturation. This process marks a critical checkpoint in T-cell development.

A productive rearrangement results in a functional TCR that must interact with major histocompatibility complex (MHC) molecules presented by cortical epithelial cells for survival.

Central Tolerance Induction

The interaction between double-positive thymocytes and MHC molecules is vital; failure to engage leads to apoptosis through a process known as "death by neglect."

Positive selection occurs when thymocytes successfully recognize self-MHC molecules, determining whether they will express CD4 or CD8 based on their interactions during this phase.

Final Stages of Development

Only a subset of thymocytes survives positive selection; these selected cells migrate towards medullary regions where further maturation occurs.

T Cell Development and Activation

Thymocyte Migration and Selection Processes

Thymocytes migrate from the thymic cortex to the medulla, where they express various tissue-specific proteins. The transcription factor Aire is crucial for regulating genes expressed in other tissues.

Thymocytes that receive strong signals through their TCR (T Cell Receptor) are deemed autoreactive and undergo negative selection, leading to cell death.

A double-positive thymocyte must be positively selected in the cortex before migrating to the medullary zone, successfully passing through negative selection without receiving intense signals.

Surviving thymocytes exit the thymus as mature single-positive T cells, capable of recognizing self-MHC molecules specific to each individual.

Not all self-proteins enter the medullary zone; autoreactive T cells are controlled by mechanisms known as peripheral tolerance induction.

Ontogeny of Lymphocytes

The mechanisms involved in lymphocyte ontogeny include positive and negative selection processes that allow T cells to emerge with antigen receptors for unknown antigens.

Mature naïve T cells traffic through secondary lymphoid organs, where only professional antigen-presenting cells can activate them upon encountering their specific antigens.

Dendritic Cell Activation and Function

Immature dendritic cells in tissues can be activated by pathogen-associated molecular patterns (PAMPs) and pro-inflammatory cytokines, enhancing their ability to present antigens.

Activated dendritic cells express chemokine receptors that guide them via lymphatic vessels to secondary lymphoid organs, positioning themselves strategically within these areas for optimal interaction with naïve T cells.

Interaction Between Naïve T Cells and Dendritic Cells

Naïve T cells entering secondary lymphoid organs search for specific peptide-MHC complexes on dendritic cells; if not found, they will leave the node and continue circulating.

Upon finding a matching complex on a dendritic cell's surface, naive T cells initiate activation processes which involve multiple signaling pathways leading to proliferation and differentiation.

Requirements for Naïve T Cell Activation

For activation, naïve T cells require two signals: recognition of peptide-MHC complexes by their TCR (first signal), followed by co-stimulatory signals involving CD28 interacting with CD80/CD86 on mature dendritic cells (second signal).

Successful reception of both signals stabilizes IL-2 messenger RNA production; activated naïve T cells begin producing IL-2 while expressing high-affinity IL-2 receptors necessary for further proliferation.

T-Cell Activation and Dendritic Cell Interaction

Clonal Expansion of T-Cells

The process begins with high affinity clonal expansion of T-cells due to IL-2 production, which is a result of signals received from dendritic cells.

Reciprocal Activation Between Dendritic Cells and T-Cells

Activated T-cells express CD40L, interacting with CD40 on dendritic cells, enhancing the latter's ability to stimulate T-cell responses.

Co-stimulation Requirements for T-Cell Activation

CD4+ and CD8+ naïve T-cells require recognition of MHC-peptide complexes along with co-stimulatory signals for activation; however, CD8+ T-cells need higher levels of these signals compared to CD4+.

Importance of Dendritic Cell Interactions

Naïve CD8+ T-cells often depend on prior interaction between dendritic cells and activated CD4+ T-cells to enhance their co-stimulatory capacity for effective activation.

Consequences of Inadequate Co-stimulation

If naïve T-cells fail to receive adequate co-stimulatory signals after recognizing antigenic peptides, they become anergic—losing the ability to activate and eventually undergoing apoptosis.

Migration Patterns of Activated T-Cells

Chemokine Receptor Expression in Naïve vs. Activated T-Cells

Naïve lymphocytes migrate through secondary lymphoid organs due to specific chemokine receptors; upon activation, they alter adhesion molecule expression patterns allowing access to peripheral tissues.

Role of Dendritic Cells in Tissue-Specific Responses

The timing and location of naïve T-cell activation by dendritic cells influence their migration profile towards specific tissues based on released substances during activation.

Differentiation into Effector Cells

Influence of Tissue-Derived Signals on Effector Functionality

For instance, intestinal dendritic cells release retinoic acid that prompts effector or memory T-cells to express specific adhesion molecules facilitating access to the intestinal mucosa.

Skin-Derived Signals Affecting Memory Cell Migration

Conversely, skin-derived dendritic cells produce calcitriol (active Vitamin D3), influencing effector memory cell patterns for accessing skin tissue post activation.

Mechanisms of Cytotoxicity in Effector T-Cells

Activation Timeline Post Antigen Encounter

The complete process from activation through clonal expansion and differentiation into cytotoxic effector cells takes approximately five days following antigen exposure.

Targeting Infected Cells

Once activated, cytotoxic effector T-cells seek out infected host cells using adhesion molecules like LFA1 for effective targeting based on previous encounters with antigens presented by dendritic cells.

Mechanisms of Cytotoxic T Cells and Their Activation

Cytotoxic Functions of CD8 T Lymphocytes

CD8 cytotoxic T lymphocytes can perform effector functions such as producing pro-inflammatory cytokines and inducing apoptosis in target cells upon receiving activation signal 1.

There are two cytotoxic mechanisms employed by CD8 T lymphocytes: a non-secretory mechanism involving direct interaction with target cells, and a secretory mechanism that releases perforins and granzymes.

Comparison with Natural Killer (NK) Cells

Unlike NK cells, which activate based on a balance of activating and inhibitory signals, CD8 T lymphocytes recognize specific antigens presented on target cells to initiate their cytotoxic response.

Upon recognizing the antigenic complex on the target cell, activated CD8 T lymphocytes receive activation signal 1, leading to the induction of apoptosis in the target cell.

Differentiation Pathways for Naive CD4 T Lymphocytes

Naive CD4 T lymphocytes have significant potential to differentiate into various effector profiles depending on cytokine environments encountered during their activation.

The differentiation process is influenced by interactions with dendritic cells in secondary lymphoid organs, where cytokine presence determines whether they become Th1 or Th17 effector cells.

Profiles of Effector CD4 T Lymphocytes

Th1 and Th17 profiles are characterized by specific cytokine production; Th1 produces interferon-gamma while Th17 is involved in responses against bacterial and fungal infections.

The role of Th1 cells includes immunity against intracellular microorganisms and antiviral responses.

Factors Influencing Differentiation into Various Subtypes

The presence of IL-21 during naive CD4 T cell activation favors differentiation towards follicular helper T (Tfh) cells, which assist B cell activation.

In contrast, high levels of IL-4 promote differentiation towards the Th2 profile, which is crucial for combating helminth parasites and allergic diseases.

Regulatory Mechanisms in Immune Response

Naive CD4 T lymphocyte differentiation can also lead to regulatory profiles under certain conditions; IL-10 promotes regulatory type 1 (Tr1), while transforming growth factor-beta (TGF-beta) leads to Tr3 regulatory cells.

These regulatory subsets play essential roles in modulating immune responses to prevent overactivity that could lead to autoimmunity.

Complexity of Immune Responses

During immune challenges like infections or tumors, multiple effector profiles may coexist; for instance, both Th1 responses and antibody production can occur simultaneously.

Cytokines produced by one type of effector cell can inhibit the differentiation pathways of others; for example, IL-4 inhibits Th1 development while interferon-gamma suppresses Th2 differentiation.

Transcription Factors Governing Differentiation

Different transcription factors regulate the expression patterns associated with each effector profile. For instance:

Th1 requires transcription factors like TBET and STAT4 for interferon-gamma stimulation.

Th2 differentiation involves GATA3 suppression through these same factors.

Inflammatory Response and Immune Activation

Role of Macrophages in Inflammation

Activated macrophages play a crucial role in mediating inflammatory responses in peripheral tissues, with interferon gamma and IL-2 being key factors that activate NK cells and promote CD8 T cell activation.

Mycobacterium tuberculosis and Tuberculosis

Mycobacterium tuberculosis is the causative agent of tuberculosis, primarily affecting the lungs but can also impact other organs. It spreads through airborne bacteria expelled by infected individuals during coughing.

According to a 2019 WHO report, tuberculosis is one of the top ten causes of mortality worldwide, with approximately one-quarter of the global population infected.

Immune Response to Mycobacterial Infection

Upon reaching pulmonary alveoli, macrophages and dendritic cells ingest mycobacteria, leading to the release of pro-inflammatory cytokines that recruit more leukocytes.

Specific Th1 lymphocytes activate macrophages to control mycobacterial growth within granulomas, which contain the pathogen effectively for years.

Granuloma Formation and Latent Infection

Granulomas form as a containment strategy against mycobacteria; however, if immune surveillance weakens (e.g., due to corticosteroids), granulomas may liquefy, releasing bacilli into airways.

Th17 Cells and Their Functions

Role in Autoimmunity and Extracellular Infections

Th17 cells are central not only in autoimmune phenomena but also in immunity against extracellular bacterial infections and fungi.

Th2 Cells: Key Players Against Parasites

Collaboration with B Lymphocytes

Follicular helper T cells collaborate with naïve B lymphocytes by providing secondary activation signals essential for humoral responses.

Immune Response Against Allergens

Th2 cells are critical for immune responses against extracellular parasites; they induce mast cell degranulation and stimulate mucus secretion to expel parasites.

Cytokine Production by Th2 Cells

Mechanisms Supporting Antibody Production

Cytokines produced by Th2 cells facilitate antibody class switching towards IgE production specific for pathogens, enhancing opsonization for destruction via antibody-dependent cellular cytotoxicity.

Tissue Repair and Allergy Development

Understanding Regulatory T Cells and Immunological Memory

Role of Regulatory T Cells

The discussion begins with the role of Th2-type responses, highlighting the reproduction of mast cells and the functions of regulatory T cells (Tregs) in maintaining homeostasis and tolerance by limiting autoreactive clones in peripheral tissues.

Natural regulatory T cells are identified as CD4+ Foxp3+ lymphocytes that migrate directly from the thymus, possessing regulatory capabilities. Inducible regulatory T cells, on the other hand, differentiate from CD4+ T cells upon recognizing antigens in secondary lymphoid organs.

Immune Response Dynamics

The seminar focuses on understanding immunological memory generation during primary infections, illustrating phases such as innate immune response induction and adaptive immune response development leading to effector T cell generation.

It is emphasized that immunological memory ensures a faster and more efficient response upon re-exposure to the same pathogen. A graph illustrates how specific T cell numbers increase after initial exposure.

Memory Cell Generation

Following an initial antigen challenge, effector lymphocytes peak before undergoing contraction; remaining cells differentiate into memory T cells. This process enhances future responses to subsequent encounters with the same pathogen.

The importance of generating immunological memory is reiterated, showing how repeated exposures can lead to increased populations of memory T cells capable of rapid activation and differentiation into effector cells when encountering pathogens again.

Activation Mechanisms

Memory T cells can proliferate in response to pro-inflammatory cytokines like IL-12 or type I interferons even in the absence of their specific antigen. This highlights their adaptability during various infections.

Two main types of memory T cells are discussed: central memory (similar to naive lymphocytes with L-selectin expression for migration) and effector memory (lacking L-selectin but resembling effector T cell migration patterns).

Resident Memory Cells

Effector resident memory T cells can establish stable populations within tissues, acting as sentinels against reinfection. Their ability to respond quickly upon re-exposure is crucial for effective immunity.

The dynamics between activated memory T cells and their roles during reinfection are explored, emphasizing how they reside in tissues ready to respond rapidly without needing full systemic activation.