Figure 1. Contrasting conventional T-cell activation with polyclonal in vitro methods. In response to peptides presented with major histocompatibility complex (MHC) on the surface of an antigen-presenting cell (APC), a small number of T cells will engage the T-cell receptor (TCR) and transduce a signal that acts in concert with signals from other costimulatory molecules (cos) to induce interleukin 2 (IL-2) receptor expression, and IL-2 to be secreted. A similar sequence of events can be induced without the unique, limiting participation of a peptide. Superantigens bridge the peptide vacant MHC of an APC to the TCR. Allo-MHC presents a sufficient range of novel MHC–peptide combinations that a high frequency of T cells can be stimulated. Plant lectins bind and crosslink glycoproteins on the T-cell surface involved in activation. Oxidation leads to the permanent chemical crosslinking of cell surface glycoproteins and subsequent uncontrolled receptor triggering. Antibodies directed against the TCR, CD3 or costimulatory receptors can crosslink the target molecule and induce activating signals. In the membrane bypass protocol pharmaceutical agents cross the membrane and activate intracellular signalling cascades directly. Irrespective of the method used, all procedures result in IL-2-driven T-cell proliferation.

Figure 2. Three modes of B-cell activation in vitro. (1) When activated conventionally by the T-independent 1 (TI-1) stimulus lipopolysaccharide (LPS), B-cell antigen receptors (BCRs) help to focus the mitogen to the cell surface and thereby trigger additional activating receptors. Antigen receptor stimulation, however, will inhibit immunoglobulin secretion. High LPS concentrations in vitro bypass the need for any involvement of surface immunoglobulin and induce rapid proliferation and cytokine-independent immunoglobulin secretion. (2) TI-2 antigens form crosslinking lattices with the antigen receptor of specific B cells. The signals generated synergize with cytokines to induce proliferation and immunoglobulin secretion. These physiological signals are commonly reproduced in vitro using anti-immunoglobulin antibodies. (3) The conventional pathway of T-dependent (TD) activation involves a sequential dialogue between T and B cells. The B cell first captures antigen, internalizes it and traffics degraded peptides to the surface in association with the MHC class II. (2) Specific T cells are activated and, in response to the stimulus, express cell surface molecules, including the ligand for CD40, de novo. In addition, the T cell secretes cytokines (3). In response to both CD40 engagement and signalling from cytokine receptors, the B cell proliferates and secretes immunoglobulin (4). These processes can be shortcircuited by engaging CD40 directly with CD40 ligand or anti-CD40 antibodies and adding cytokines to culture exogenously.