Synergistic roles of IgM and complement in antigen trapping and follicular localization

We have investigated the roles of IgM and complement (C) in the enhancement of primary immune responses and the localization of protein antigen (Ag) in the spleen. Pentameric but not monomericIgM enhances antibody (Ab) responses in both wild‐type and secretory μ‐deficient (μs–/–) mice, indicating that a single IgM clone is sufficient as long as it activates C. Ag localizes on follicular dendritic cells (FDC) within 16 h after injection of immune complexes (IC) containing pentameric but not monomeric IgM. Surprisingly, pentameric IgM‐containing IC were trapped in spleens of C3‐depleted and Cr2‐deficient mice. However, the IC were found only in the marginal zone (MZ), associated predominantly with MZ macrophages. IC were also detected in the MZ in normal mice within 1 h after injection, but associated with other cells in addition to MZ macrophages. The results demonstrate an important role for pentameric IgM in the initiation of Ag trapping, a step independent of C3 activation and of the interaction of IC with CR1 and CR2. The data also provide direct evidence that C3 activation is required for the next phase of localization, in which Ag moves from the MZ to FDC, with consequent enhancement of specific immune responses.