Synthesis of multiple immunoglobulin classes by single lymphocytes.

There is good evidence that, in agreement with the principles of clonal selection, the immunoglobulin molecules synthesized by one B immunocyte, or by a clone of these cells, are uniform with regard to the Fv portion; that is, they have a uniform antibedy-combining site and a uniform idiotype (reviewed by Makela and Cross 1970). This condition applies beth to the membrane-bound immunoglobulins (Raft et al. 1973) and to the secreted molecules (Miikel~i 1968). Thus there are predictable restrictions to the synthesis of immunoglobul-in chains of one allotype only (allelic exclusion) since the two homologous chromosomes are not expected to carry identical sets of variable-region genes, neither germ-line genes nor genes generated by a process of somatic mutation. The same reasoning applies for normal B immunocytes, which are restricted to the synthesis of only one type of light (L) chain (K or ~,) because the two types of L chains, coded by genes on different chromosomes, do not have identical sets of variableregion genes (see Pernis 1968). On the other hand, the synthesis by a single immunocyte, or by a clone, of more than one class of heavy (H) chain is not in contrast with the principle of the uniformity of the antibody-combining site because the constant-region H-chain genes share the same set of V. genes. I t is possible, therefore, that once a given V. gene has been chosen by a cell it may become associated with more than one constant-region gene on the same chromosome by successive translocations of the VH gene itself (Gally and Edelman 1970) or by the successive excision of the DNA region between Vs and the different C. genes (Tonegawa et al., this volume). These models predict that different H-chain classes may be synthesized by one clone of B immunocytes, a phenomenon for which there is ample experimental evidence; but if only one chromosome is active, a single cell should not simultaneously synthesize more than one class (except of course for a period corresponding to the persistence and function of previously synthesized mRNA). This problem concerning the genetic control of immunoglobulin synthesis was one reason why we were led to consider the evidence for the simultaneous synthesis of more than one H chain by single lymphocytes; another reason was that if two classes of immunoglobulins are regularly synthesized by single lymphocytes, particularly as membrane receptors, this probably implies an important physiological function for the simultaneous presence of both classes of receptors. In fact, there are reports in the l i terature (Takahashi et al. 1969; Litwin et al. 1973; van Boxel and Buell 1974) of synthesis of more than one H chain by single cells in human lymphoblastoid lines cultured in vitro. These observations, however, might be explained on the basis of the chromosomal abnormalities often found in these cells (see, for instance, Takahashi et al. 1969) and, although interesting in themselves, might not be very meaningful with respect to the biological problems indicated above. One clear case of the synthesis of multiple classes of immunoglobulins by single immunocytes being explained by an abnormal choromosomal set is in lines obtained by the fusion of different plasmacytoma cells between themselves or with normal immunocytes (K6hler and Milstein 1975). Of course in this case, the restrictions pertaining to the synthesis of one L-chain type and one H-chain variable region are also abrogated. With this in mind, we will consider in this report mainly the evidence for multiple H-chain synthesis by single normal lymphocytes. The evidence from lymphoblastoid lines or from lymphomas should always be considered with a view to possible chromosomal abnormalities due to cell fusion or chromosome nondisjunction. It is interesting to point out, however, that the latter case would still be compatible with the synthesis of different H chains with the same variable region (idiotype); the finding of only one L chain would not clearly disprove either cell fusion or chromosome nondisjunction. The most common associations of two different H chains detected in single normal immunocytes are g plus 8 (Rowe et al. 1973; Knapp et al. 1973) and g plus 7 chains (Pernis et al. 1971). In this report, we will consider the evidence in favor of an actual simultaneous synthesis of these chains by the cells.