Macrophage dependency of in vitro B cell response to influenza virus antigens

Specific B cell responses to influenza viruses were induced by in vitro stimulation of spleen cells from normal or influenza-immune mice with inactivated influenza virus. Both primary and secondary responses to A/Pt Chalmers/73 (A/Pt Ch/73) and A/New Jersey/76 (A/NJ/76) viruses were detected in a hemolytic plaque assay, with peak numbers of specific plaque-forming cells (PFC) present after 4 days of incubation. Depletion of macrophages from BALB/c spleen cell suspensions by treatment with either carrageenan or carbonyl iron markedly reduced the primary and secondary B cell responses to influenza virus. The responses could be restored by reconstitution with either BALB/c or CBA/J macrophages. Specificity of the antibody response was demonstrated in cross-reaction experiments with A/Pt Ch/73 and A/NJ/76 viruses. Evidence of recall to A/NJ/76 virus was, however, found in A/NJ/76-immune spleen cells stimulated in vitro with A/Pt Ch/73 virus. Greatest in vitro stimulation of A/Pt Ch/73-immune spleen cells was produced with inactivated whole A/Pt Ch/73 virus preparations; specific responses were low or absent after stimulation with preparations of partially disrupted virus vaccine, a mixture of purified hemagglutinin (HA) and neuraminidase antigens or soluble hemagglutinin containing similar amounts of viral HA antigen. This lack of response to A/Pt Ch/73 HA could not be overcome with A/NJ/76 whole virus together with purified A/Pt Ch/73 HA.