1382 Restimulation of Secretion by Mouse

Production of mouse h u m a n hybrids by fusion of mouse myeloma cells with human lymphoid cells has been well documented since the early studies of Schwaber and Cohen (1, 2). These hybrids do, however, lack stability and display a preferential loss of h u m a n chromosomes in the days after fusion (3) in a manner that appears to be nonrandom (4, 5). Their chromosomal instability has proved valuable for gene mapping studies, including experiments that have resulted in the provisional assignment of human heavy chain genes to chromosome 14 (5-7). On the other hand, the preferential loss of h u m a n chromosomes has made it difficult to use the hybrid lines for sustained production of human immunoglobul in (Ig) (8, 9). Moreover, the problem has not yet been solved by substituting myeloma cell lines of h u m a n origin and, a l though several groups have successfully produced h u m a n h u m a n hybrids (10 12), long-term antibody-secreting cell lines have not to our knowledge been reported. We describe here three mouse-human lines derived from a single fusion between the mouse myeloma line NS-1 and h u m a n tonsillar lymphocytes. One of these lines has continued to secrete human Ig over a 2-yr period in culture. The other lines were less stable and eventually stopped secreting Ig. Nonsecreting lines were examined using isoenzyme assays to determine whether chromosome 14 had been lost. Because the majority of nonsecreting hybrids still had chromosome 14, and therefore appeared to retain the structural genes for h u m a n Ig, they were stimulated with mitogens in an a t tempt to regain Ig secretion. Lipopolysaccharide (LPS) 1 st imulation of three nonsecreting cell lines resulted in sustained Ig production, indicating that the cessation of Ig secretion in those hybrid cells may be caused by factors other than loss of structural genes. The ability to regain Ig secretion through mitogen stimulation suggests that mouse-human hybridomas may still be of potential value as a source of h u m a n monoclonal antibody.