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Session 15: Memory

The study of CD4 T cell memory has been limited by the difficulties in tracking small populations of antigen specific cells. Here we use MHC class II tetramers to follow memory cells for over 200 days postimmunisation. The importance of this system lies in the physiological number of precursor cells, providing a more realistic system to study CD4 T cell immunology than T cell receptor transgenic studies. The phenotype of the memory population has been examined; the cells are homogeneously CD44 high CD62L low, and IL-7Ralpha positive, and overtime, these cells decline slowly. The recall response has also been investigated and found to be faster and of greater magnitude than the primary response. Moreover, a qualitative difference in the requirements for memory cell activation compared to that for naive cells has been found in respect to CD40-CD40L co-stimulation. By transferring Ly5.1 congenic memory cells into C57BL/6 mice this recall response can be examined in more detail, and the contribution of naive cells to the secondary response evaluated. Class II tetramers provide an invaluable tool in dissecting the signals involved in the survival of memory cells in the competitive environment of the intact immune system.