Cells producing low-molecular-weight antibody to Escherichia coli lipopolysaccharide.

CD-1 mice immunized with sheep red blood cells (SRBC) or Escherichia coli lipopolysaccharide (LPS), developed splenic plaque-forming cells (PFC) producing low-molecular-weight antibody; these cells were detected by means of purified rabbit antisera to mouse gamma(1), gamma(2a), and gamma(2b) immunoglobulins. In contrast to SRBC, the primary gamma(1) response to LPS was absent and gamma(2a) and gamma(2b) PFC were detected irregularly. Both immunogens elicited a secondary cellular response in all three subclasses without a corresponding increase in "direct" or gamma(M) PFC. An increase in serum bactericidal antibody following a second injection of LPS was not parallelled by an increase in splenic gamma(M) PFC; it might therefore involve the synthesis of gamma(2) complement-fixing antibody.