Part 1: Immunological Tolerance
The first report that arose from the workshop focuses on immunological tolerance. We first outline the major ideas we thought sufficiently plausible to provide a context for discussing more controversial issues around tolerance. We then report on our discussion of different experiments that appear paradoxical in terms of the different, contemporary models of CD4 T cell inactivation/ activation, and how such observations might be resolved in terms of insights provided by these contemporary models.
These discussions bear on the plausibility of the Pathogen-Associated Molecular Pattern (PAMP), Danger and Two Step, Two Signal Models for the activation of na€ıve CD4 T cells. Some of the observations considered appear paradoxical in terms of the PAMP and Danger Models, but not with the Two Step, Two Signal Model. For example, genetically immunodeficient mice have been given foreign, sterile ectopic grafts, and the immune system allowed to develop once these grafts were well-healed in, and so in the absence of PAMPs or danger. The grafts were rejected, unexpected on the PAMP or Danger Models.
We also discussed considerations and observations bearing on the widely held idea that antigen must crosslink the membrane Ig receptors of a B cell to initiate the generation of signal 1, or the alternative possibility that monovalent binding by antigen can do so. We favored the latter possibility, and discussed a particular model, “the Elbow Model,” for how this might be achieved.
Read the full report here
Part 2: Immune class regulation and its medical significance
We discussed different proposals for how the nature of the Th1/Th2 phenotype of an immune response is determined, and favored one, the Threshold Hypothesis, as plausible and so useful as the basis for further discussions. The activation of a target CD4 T cell can be facilitated by helper CD4 T cells when the CD4 T cells interact via an antigen-presenting cell (APC). The Threshold Hypothesis states that tentative and robust antigen-mediated CD4 T cell cooperation results in the target CD4 T cell respectively giving rise, upon activation, to Th1 and Th2 cells.
We primarily discussed four topics. We briefly discussed in the background section certain limitations of the Th1/Th2 paradigm in understanding immune class regulation, and the remarkable anti-inflammatory properties of human IgG4 antibody. Secondly, we assessed the role of class II MHC molecules in determining the number of mature CD4 T cells and so affecting the Th1/Th2 phenotype of immune responses.
We also discussed the controversial role of CD8 T cells in affecting the Th1/Th2 phenotype of responses to MHC and other antigens, and the potential role of their relative scarcity in neonates in biasing responses towards an antibody, Th2 mode.
Lastly, we examined the regulation of the Th1/Th2 phenotype of both primary and on-going immune responses in the context of the intriguing proposal that antigen initially generates different classes/subclasses of immunity and then selects, by a feedback mechanism, the most effective class. We found this interesting idea difficult to reconcile with various observations.