Class I HLA oligomerization at the surface of B cells is controlled by exogenous beta sub(2)-microglobulin: implications in activation of cytotoxic T lymphocytes

Submicroscopic molecular clusters (oligomers) of class I HLA have been detected by physical techniques [e.g. fluorescence resonance energy transfer (FRET) and single particle tracking of molecular diffusion] at the surface of various activated and transformed human cells, including B lymphocytes. Here, the sensitivity of this homotypic association to exogenous beta sub(2)-microglobulin ( beta sub(2)m) and the role of free heavy chains (FHC) in class I HLA oligomerization were investigated on a B lymphoblastoid cell line, JY. Scanning near-field optical microscopy and FRET data both demonstrated that FHC and class I HLA heterodimers are co-clustered at the cell surface. Culturing the cells with excess beta sub(2)m resulted in a reduced co-clustering and decreased molecular homotypic association, as assessed by FRET. The decreased HLA clustering on JY target cells (antigen-presenting cells) was accompanied with their reduced susceptibility to specific lysis by allospecific CD8 super(+) cytotoxic T lymphocytes (CTL). JY B cells with reduced HLA clustering also provoked significantly weaker T cell activation signals, such as lower expression of CD69 activation marker and lower magnitude of TCR down-regulation, than did the untreated B cells. These results together suggest that the actual level of beta sub(2)m available at the cell surface can control CTL activation and the subsequent cytotoxic effector function through regulation of the homotypic HLA-I association. This might be especially important in some inflammatory and autoimmune diseases where elevated serum beta sub(2)m levels are reported.