IMMUNOLOGY - Roitt I. - Mir 2000

Chapter 14. Immunological Tolerance

TOLERANCE INDUCED IN VITRO

Tolerance of B AND T Cells can readily be induced in vitro

Antigens capable of cross-linking B-Cell immunoglobulin receptors without possessing mitogenic properties can induce B-cell tolerance. High concentrations of antigen are required to render mature B lymphocytes tolerant, whereas immature B cells are susceptible to the tolerogenic effects of antigen even at low concentrations. Cross-linking of B-cell immunoglobulin receptors can be achieved using anti-IgM Monoclonal Antibodies. High concentrations of such antibodies inhibit the division of pre-B cells, preventing their further differentiation into B cells bearing membrane-bound IgM (mIgM⁺). Conversely, lower concentrations of these antibodies allow pre-B cells to differentiate into morphologically normal B lymphocytes with a normal number of immunoglobulin receptors, albeit rendered profoundly anergic (Fig. 14.12). Thus, both B-cell function and numbers can be modulated by acting on immunoglobulin receptors during the critical developmental window when they first appear on The Cell surface (the transition from pre-B to B cells).

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Fig. 14.12. Surface expression of membrane-bound IgM (mIgM) and functional activity (reactivity to lipopolysaccharide, LPS) are shown as percentages of normal levels. It can be seen that reducing mIgM expression requires approximately 10,000-fold higher concentrations of anti-IgM antibodies than are needed to suppress functional activity to the same degree. Even at 10^-3 mcg/mL, anti-IgM antibodies significantly reduce LPS reactivity without affecting mIgM expression.

Human T cells express class II MHC molecules and can mutually present antigenic Peptides to one another. Depending on The Nature of the T cells and the antigen, this interaction results either in tolerance due to reduced receptor expression and anergy, or in activation. Studies on mouse T-cell clones have demonstrated that activation in the absence of costimulatory signals inevitably leads to anergy. Presentation of peptide antigens by antigen-presenting cells (APCs) that have been chemically fixed, or by Artificial Membranes containing class II MHC molecules, induces anergy—much like the activation of T cells by anti-TCR antibodies immobilized on a plastic surface. Apparently, mere binding of the TCR without accompanying costimulation results in anergy.

The Effect of antigen—clonal abortion of B cells, deletion of mature B cells, or anergy—depends on its valence and cross-linking capacity

The outcome of a B cell's encounter with an antigen depends on several factors: antigen valence (i.e., the number of epitopes per molecule), antigen concentration, receptor affinity for the relevant epitopes, and the cell's maturation stage. At one end of the spectrum is the clonal deletion of less mature cells, which is most likely for cells bearing high-affinity receptors that encounter high-valence antigens or antigens present at high concentrations (Fig. 14.13). The opposite end of the spectrum is a lack of effect at low antigen concentrations and low affinity. Clonal anergy occupies an intermediate position among the possible outcomes.

✵ Clonal abortion: a polyvalent antigen at an appropriate concentration can delete immature B cells, preventing their further differentiation; pre-B cells exhibit a high degree of susceptibility to tolerance.

✵ Clonal deletion: very strong negative signals can lead to the elimination of mature B cells.

✵ Clonal anergy: at intermediate concentrations of a polyvalent antigen, B cells differentiate into morphologically normal cells with a normal number of immunoglobulin receptors, but are driven into a state of profound anergy.

✵ Clonal ignorance: the antigen has no effect on B cells if its concentration is too low or the affinity of the antigen receptor is exceedingly weak; this represents a passive form of immunological tolerance.

✵ Blockade of antibody-forming cells (AFCs): an excess of T-independent antigen blocks antibody secretion by AFCs; the latter possess low susceptibility to tolerance.

Fig. 14.13. As immature B cells differentiate into antibody-forming cells (AFCs), their resistance to tolerance induction progressively increases. The type of tolerance that ensues depends on the cell's maturation stage and the strength of the antigenic signal. The latter, in turn, is determined by the affinity of the antigen receptor for the relevant epitopes, as well as the concentration and valence of the antigen contacting the cell.

A second window of heightened cellular sensitivity to tolerance induction occurs transiently during the GENERATION OF B-cell immunological memory

Secondary B cells (derived from memory B cells generated via T-dependent stimulation; see Chapter 11) are highly susceptible to the tolerogenic effects of polyvalent antigen epitopes in the absence of T-cell help (Fig. 14.13). The existence of this stage of heightened sensitivity to tolerance induction likely serves to eliminate newly emerging memory B cells that have acquired autoreactivity (As a result of somatic hypermutation).