Section C – The adaptive immune system
T cells and B cells mature in the thymus and bone marrow, respectively. In the resting state both T and B lymphocytes have a similar morphology with a small amount of cytoplasm (Fig. 1). They have specific but different antigen receptors and a variety of other surface molecules necessary for interaction with other cells (Table 1). These include molecules required for their activation and for movement into and out of the tissues of the body. This ability to migrate into the tissues and return via the lymphatic vessels to the bloodstream (recircu- lation) is a unique feature of lymphocytes.
Fig. 1. A blood lymphocyte. Reproduced from Immunology 4th edn, Roitt, Brostoff and Male, with permission from Mosby.
Table 1. Characteristics of human B and T cells
T cells B cells
Site of maturation Thymus Bone marrow
Antigen receptor TCR Antibody
Requirement of MHC for recognition Yes No
Characteristic ‘markers’ All have TCR, CD3 Surface Ig, CD19, CD20, CD21
Th – CD4 CD79
Tc – CD8
Main location in lymph nodes Paracortical area Follicles
Memory cells Yes Yes
Function Protect against intracellular microbes Protect against extracellular Provide help for Ab responses microbes
Products Th1 – IFNγ, TNFα Antibodies (B cells mature into
Th2 – IL-4, IL-5, IL-6 plasma cells) Tc – Perforins
There are two classes of T lymphocytes, T helper (Th) cells and T cytotoxic (Tc) cells. All T lymphocytes have antigen receptors (TCR) (Topic F2) that deter- mine their specificity and CD3, which is essential for their activation (Topic F4).
These molecules also serve as ‘markers’ to identify T cells. B lymphocytes make and use antibodies as their specific antigen receptor. They have molecules simi- lar to CD3, i.e. CD79, which are important in their activation. B lymphocytes can mature into plasma cells that produce and secrete large amounts of anti- body.
42 Section C – The adaptive immune system
T lymphocytes T cell ontogeny
The thymus is derived from the third and fourth pharyngeal pouches during embryonic life and attracts (with chemoattractive molecules) circulating T cell precursors derived from hemopoietic stem cells (HSC) in the bone marrow. In the thymus, these precursors differentiate into functional T lymphocytes under the influence of thymic stromal cells and cytokines. In particular, in the thymic cortex the precursors (now thymocytes) associate with cortical epithelial nurse cells critical to their development. In this site there is major thymocyte prolifera- tion, with a complete turnover of cells approximately every 72 hours.
Thymocytes then move into the medulla, where they undergo further differenti- ation and selection. Most of the thymocytes generated each day in the thymus die by apoptosis with only 5–10% surviving. Molecules important to T cell func- tion such as CD4, CD8 and the T cell receptor develop at different stages during the differentiation process (Fig. 2).
C1 – Lymphocytes 43
Tc
Thymus Periphery
CD4,CD8
T cell precursor
Double positive
CD4,CD8 TCR CD3
CD4
CD8 T
Th
Fig. 2. Development of CD4+and CD8+T cells in the thymus.
Thymus function
The main functions of the thymus as a primary lymphoid organ are to: (a) produce sufficient numbers (millions) of different T cells each expressing unique T cell receptors (generate diversity) such that in every individual there are at least some cells potentially specific for each foreign antigen in our envi- ronment; (b) select T cells for survival in such a way that the chance for an auto-immune response is minimized. It is important to note that T cell develop- ment within the thymus is independent of exogenous (foreign) antigens.
Generation of T cell diversity in the thymus
Millions of T cells, each with receptors specific for different antigens, are gener- ated by gene rearrangement from multiple (inherited) germline genes. Each of the T cells produced in the thymus has only one specificity coded for by its antigen receptor.
Positive and negative selection
Once produced in the thymus, T cells undergo selection using their newly produced receptors. T cells with receptors that bind weakly to MHC molecules are selected whilst those with receptors which bind strongly to MHC and self antigens die through apoptosis (central tolerance to self, Topic G2) and are removed by phagocytic macrophages.
Mature T cells and their subsets
T cells which survive the selection process mature into functionally distinct subsets (Fig. 3). These cells migrate to the peripheral lymphoid tissues where they complete their functional maturation and provide protection against invad- ing microbes. Some T cells reside, at least temporarily, in T-cell-dependent areas of tissues. T cells can be identified using monoclonal antibodies specific for characteristic molecules such as the T cell receptor (TCR) or CD3 (Table 2).
These cells function to control intracellular microbes and to provide help for B cell (antibody) responses. Two different kinds of T cells are involved in these functions, T helper (Th) cells and T cytotoxic (Tc) cells.
44 Section C – The adaptive immune system
TCR
TCR LSC
Th Tc
T
T
γ δ
α β
Fig. 3. Development of αβand γδT cells from lymphocyte stem cells (LSC). Two types of T cells are produced in the thymus with different TCRs (αβand γδ). The classical T cells (Th and Tc) utilize αβfor their TCR.
Th cells provide help for B cells through direct cell surface signaling and by producing cytokines that are critical to B cell growth and differentiation. In addition to TCR and CD3, Th cells also express cell surface CD4 molecules that bind to MHC class II molecules, an interaction required for their activation by antigen (Topic F2). Th cells can be further subdivided into Th1 and Th2 cells based on their ability to help in the development of different immune responses (Topic F5), which is in turn related to their cytokine profiles. The average percentages of these cells in the peripheral blood are shown in Table 3.T cyto- toxic (Tc) cells mediate killing of infected cells, primarily those infected with virus. These cells express, in addition to TCR and CD3, a cell surface molecule, CD8, that binds to MHC class I and is important for these cells to interact effec- tively with virally infected cells.
The bone marrow and B cell ontogeny
B cells develop from hemopoietic stem cells primarily (perhaps exclusively) in the microenvironment of the fetal liver and, after birth, the bone marrow. The two main functions of the bone marrow as a primary lymphoid organ are to: (a) produce large numbers of B cells, each with unique antigen receptors (antibod- ies) such that, overall, there is sufficient B cell diversity to recognize all of the antigens in our environment (generate diversity); (b) eliminate B cells with anti- B lymphocytes
and plasma cells
gen receptors for self molecules. The early stages of B cell development (like that of T cells) is independent of exogenous antigen. Mature B cells leave the bone marrow and migrate via the bloodstream to the secondary lymphoid organs/tissues where they can be found in loose aggregates (primary follicles) in lymphoid tissues or in well-defined proliferating foci (germinal centers).
Two kinds of B cells (B1 and B2) have been identified. The B2 cells are produced in the bone marrow (conventional B cells) as described and with the help of Th cells produce IgG, IgA and IgE antibodies. However, B1 cells arise
C1 – Lymphocytes 45
Table 2. Surface receptors on T cells
Surface molecules Function
The T cell receptor complex
TCR Antigen specific receptor (most T cells utilize αβdimers; some use γδ dimers
CD3 (γ,δ,εand ζ (zeta) chains) Signaling complex associated with the TCR: mediates T cell activation on binding of TCR to MHC–peptide complexes Subset markers
CD4 (on helper T cells) Binds to MHC class II molecules and restricts Th cells to recognizing only peptides presented on MHC class II
CD8 (on cytotoxic T cells) Binds to MHC class I molecules and restricts Tc cells to recognizing only peptides presented on MHC class I
Co-stimulatory molecules
CD28 Binds to CD80/CD86 on B cells and APC and positively regulates T
cell activation
CTLA4 Binds to CD80/CD86 on B cells and APC and downregulates T cell
activation
CD154 (CD40L): on activated Th cells Binds to CD40 on B cells and APC: triggers activation of APC and activation and antibody class switching of B cells
Adhesion molecules
LFA-1 Binds to ICAM-1 and facilitates interactions with other cells including B cells, APCs and target cells
CD2 (LFA2) Binds to LFA-3 and facilitates interactions with other cells including B cells, APCs and target cells
CD45RA (on naïve T cells) Involved in signal transduction CD45R0 (on activated/memory T cells) Involved in signal transduction
Table 3. Human peripheral blood lymphocyte populations
T cells B cells NK cells
Th Tc
Percent of lymphocytes 55 25 10 10
Functional properties Antigen specific, produce Antigen specific, produce Mediate ADCC, tumor cytokines, memory cells, cytokines, memory cells, surveillance, no memory, lyse effector cells plasma cells (antibody virus-infected cells and tumor
factories) cells lacking MHC class I
early in ontogeny, express mainly IgM antibodies encoded by germline anti- body genes, mature independently of the bone marrow and generally recognize multimeric sugar/lipid antigens of microbes and are thymus independent (Topic E2).
Generation of antigen receptor diversity and negative selection of B cells Antibodies, like T cell receptors, are encoded by multiple genes. These genes, which are distinct from the T cell antigen receptor genes, rearrange during the pro-B cell stage to create a unique cell surface receptor that defines its speci- ficity for antigen (Topic D3). Since rearrangement occurs in millions of different ways in these developing cells, many B cells, each with a different specificity, are generated. This generation of diversity occurs in the absence of foreign protein and yields large numbers of mature B cells, at least some of which have specificity for each foreign substance or microbe. B cells with specificity for self antigens are induced to die by apoptosis (negative selection) during their imma- ture stage, i.e. when they have expressed IgM on their cell surface, but before expression of IgD. As in the thymus, the majority of the B cells die during development as a result of their production of antigen receptors that cannot be assembled or that are directed against self antigens.
Activated B cells and plasma cells
When activated by antigen and, in most cases, with T cell help, B cells (Table 4) proliferate and mature into memory cells or plasma cells. Memory cells only produce antibody for expression on their cell surface and remain able to respond to antigen if it is reintroduced. In contrast, plasma cells do not have cell surface antibody receptors. Rather, these cells function as factories produc- ing and secreting large amounts of antibody of the same specificity as the anti- gen receptor on the stimulated parent B cell. The morphology of a plasma cell (Fig. 4) is consistent with its primary function – high-rate glycoprotein (anti- body) synthesis. This includes extensive endoplasmic reticulum, mitochondria and Golgi apparatus. It should be noted that a plasma cell only produces anti- bodies of one specificity, one class and one subclass.
46 Section C – The adaptive immune system
Fig. 4. Ultrastructure of a plasma cell. Note the extensive rough endoplasmic reticulum for antibody production. Reproduced from Immunology 5th edn., 1998, Roitt, Brostoff and Male, with permission from Mosby.
Table 4. Surface receptors on B lymphocytes
Surface molecules Function
The B cell receptor complex
Antibody (IgM and IgD on mature B cells) B cell receptor (BCR) for antigen
CD79a/CD79b (Igα/Igβ) heterodimer Mediates cellular activation on binding of BCR to antigen Co-receptors All these molecules modulate B cell activation
CD19 Influences B cell activation
CD20 Ca ++channel
CD21 (complement receptor CR2) Binds to C3d, C3bi
CD32 (FcγRII: Fc receptor for IgG) Binds to IgG complexed to antigen
CD40 Signals B cell activation and antibody class switching after
engagement with CD40 ligand (CD154) on activated T cells Molecules required for T cell activation
MHC class II molecules Present peptides to Th cells
CD80/86 (B7-1,2) Binds to CD28 on T cells to trigger their activation Adhesion molecules
ICAM-1 Binds to LFA-1 and facilitates interaction with T cells
LFA-3 Binds to CD2 and facilitates interaction with T cells
C1 – Lymphocytes 47
Section C – The adaptive immune system