An immunoassay differentiating sera with antibodies to sm alone, antibodies to sm/RNP complex, and antibodies to RNP alone

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Abstract
Combined DEAE‐Sephacel and hydroxyapatite chromatography resulted in a separation of free Sm antigen from the Sm/RNP complex in rabbit thymus extracts. In Western blots, the free Sm preparation contained an immunoreactive 14‐kd (D) protein, whereas the Sm/RNP complex contained, in addition to the 14‐kd protein, a 68‐kd—reactive protein and its putative degradation fragments. Ethidium bromide staining of these preparations separated by agarose gel electrophoresis showed that the Sm/RNP preparation contained RNA, but the free Sm preparation did not. These preparations were used as antigens in an enzyme‐linked immunosorbent assay (ELISA). Sera characterized as anti‐Sm only, anti‐Sm/RNP, and anti‐RNP only were assayed. A quotient (Q) was derived from the ELISA optical density obtained with Sm/RNP as antigen divided by the optical density obtained with free Sm as antigen. Q values 12.0 in sera with anti‐RNP only. Sera with antibodies to other nuclear antigens were not reactive in this system. It has traditionally been difficult to identify sera with anti‐RNP when this is present simultaneously with, and in lower concentration than, anti‐Sm. With this method, such sera can be identified by their Q value, which falls in an intermediate region between a lower Q for anti‐Sm and a higher Q for anti‐RNP.

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