A multiparameter flow cytometric method to study surface molecules involved in interactions between subpopulations of cells.

The interactions between T and B lymphocytes are mediated by several antigen-independent adhesion molecules including LFA-1/ICAM-1 and CD2/LFA-3. Recently new pairs of adhesion molecules involved in T and B interactions have been described: CD28/B7, CD5/CD72 and CD45RO/CD22. In order to study these heterotypic adhesion events, the phenotypes of the subpopulations as well as new potential adhesion molecules involved in conjugate formation, we have developed a flow cytometric method which analyses conjugate formation between T and B cells. The two types of cells were loaded with two vital intracellular dyes: human T lymphocytes purified from blood or tonsils were labelled with BCECF-AM (green fluorescence) and the B lymphoblastoid cell line, RPMI 8866 was labelled with Indo-1-AM (blue fluorescence). The two labelled cell populations were mixed, gently centrifuged for 5 min and then incubated at 37 degrees C in a waterbath for 5 min. The cells were then gently resuspended by inversion and analysed with a double laser flow cytometer. This method permitted us to discover new molecular interactions since preincubation of the two populations with monoclonal antibodies directed against some surface molecules inhibited conjugate formation. As an example, using this technique we found that the low affinity IgE receptor, CD23 and the CR2/EBV receptor are involved in T cell/B cell adhesion and can therefore be considered as a new pair of adhesion molecules. This method also seems to be applicable to recombinant cells bearing a single adhesion molecule such as LFA-1 and ICAM-1. A particular advantage of the two intracellular dyes we used is that they are compatible with the dyes commonly used for classical simultaneous triple colour immunofluorescence (phycoerythrin and Cy-Chrome). We were thus able to determine the subpopulations involved in forming conjugates and we found that T-B conjugates were preferentially formed by CD4, CD45RO positive T cells, which are believed to be the memory T lymphocytes.