Defective Expression of β1-Integrins in Cells with Constitutively Active αLβ2-Integrins

We have investigated a potential relationship between expression of β1-integrins and adhesiveness of the β2-integrin LFA-1 (αLβ2, CD11a/CD18). By an approach of random mutagenesis and selection we established clones from the human acute lymphatic leukemia cell line HPB-ALL with (i) constitutively active LFA-1 and (ii) with no apparent integrin-β1 cell surface expression. Thirty-seven of 42 clones selected for activated LFA-1 were found to have lost apparent integrin-β1 expression. Conversely, 7 of 21 clones selected for lack of β1 expression were found to have activated LFA-1. Since this pointed toward a possible coupling between β1 expression and LFA-1 activity, we further analyzed at which level β1 expression was blocked. We focused on one clone, HAP4, with activated LFA-1 and no detectable β1 cell surface expression and found, surprisingly, that it expressed wild-type levels of β1 mRNA and, in Western blots of whole cell lysates, apparently normal levels of β1 protein. However, in addition to β1 of the expected molecular weight, HAP4 expressed a unique 48-kDa band recognized by the polyclonal anti-β1 antiserum. Immunoprecipitation experiments revealed that the epitope recognized by the anti-β1 antibody 4B4 was hidden or lost. The α4-chain was found in its precursor form but it did not associate with any β-chain, and it was not processed to its mature form. Instead α4-chains were eventually degraded. Taken together this showed that β1-chains were produced but not properly processed in HAP4. From this we propose that HAP4 is deficient in a gene product required both for proper β1 folding and for repression of LFA-1 adhesiveness.