Activity of multiple light chain genes in murine myeloma cells producing a single, functional light chain

Two cloned λ₁-producing myelomas (HOPC-1, MOPC-104E) contain rearranged κ genes and levels of mature-sized κ RNA comparable to those found in κ-producing myeloma cells. Another λ₁-producing myeloma tumor line (HOPC-2020) and a λ₁-containing B cell leukemia line (BCL₁) also contain significant levels of κ RNA. One λ₁₁-producing line (MOPC315) contains no detectable κ RNA, but it also has no κ genes in the embryonic configuration. κ-related proteins are not detectable in the λ₁-producing lines by standard procedures, but by sensitive methods at least two lines contain κ protein fragments. The MOPC-104E line produces both a 14.5K κ fragment that is not readily detectable because of its low rate of synthesis and short half-life (T_1/2 < 5 min), and a major 16.5 κ protein that lacks κ cross reactivity but is demonstrable by translation of purified MOPC104E κ RNA. The HOPC-1 κ RNA also encodes a short-lived 14K κ fragment. The MPC-11 line, which produces a mature κ RNA and protein as well as an 800 base κ fragment RNA and κ protein fragment, has both κ alleles rearranged, one apparently aberrantly between J and C_κ. Two different κ RNA species, one the same size as the MPC-11 κ fragment RNA, frequently are present in κ RNA-containing Abelson murine leukemia virus-transformed lymphoid cells as well as in 18 and 19 day murine fetal liver. For light chains, neither allelic nor isotype exclusion is generally evident in myeloma and lymphoma cells; rather both produce only a single functional light chain. Models of light chain activation must explain restriction by considering the functional properties of the light chain rather than light chain gene expression.