SummaryThe collaboration between T and B lymphocytes is used as an example to illustrate how the key features of immune regulation (cell interaction, reciprocal exchange of signals by cell contact, and dependence on soluble cytokines) serve as amplifying reactions. By linking cell-based ampli®ers in sequence, the resulting immune response is made highly sensitive to small changes in the environment. Thus, intercellular communication in the immune system can be viewed as a higher level analogue to the kinase cascades that amplify intracellular signalling mechanisms.
Immunologists have developed a range of in vitro techniques for probing the receptor mediated response of cells comprising the immune system. An important and ubiquitous method is the use of antibodies in either soluble or aggregated form to engage cell surface receptors and transmit a signal. Models of cell and molecular interactions, derived from the use of these antibodies, form the basis of our efforts to understand and explain the corresponding in vivo systems. However, interpreting in vitro experiments and distinguishing between alternative models is difficult. This complexity is illustrated here using B cell stimulation by surface immunoglobulin and CD40. The fluorescent cell labelling dye carboxyfluorescein, diacetate, succinimidyl ester (CFSE) is used to show that many anti-Ig and CD40 stimulatory agents, used to assess the role of B cells and lymphokines, are partial agonists. By modelling each step in B cell signalling, activation and division it is possible to show that small changes in signal contributed by a second receptor can generate numerous distinct dose response curves that are highly dependent on the "efficacy" of signal transmission by the primary ligand and the number of cell divisions taken in culture. Differences in dose response curves become particularly striking if the primary activating stimulus is a partial agonist. Although exemplified here with B cell stimulation the conclusions are applicable to other in vitro activation systems and suggest ways to improve both the design and interpretation of in vitro experiments.*Corresponding author. Tel.: 61 2 9565 6121. Fax: 61 2 9565 5103.
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