Adoptive transfer of immunity against virulent Semliki Forest virus with immune spleen cells from mice infected with avirulent Semliki Forest virus

Summary This paper describes the minimal requirements to protect mice adoptively against challenge with virulent Semliki Forest virus (SFV). Early immune serum, from donor mice infected with an avirulent strain of SFV, contained mainly neutralizing IgM immunoglobulins. More of these antibodies (1.80 PND₅₀ vs 0.06 PND₅₀) were needed to protect recipient mice against intraperitoneal challenge (10 LD₅₀ of SFV) than against subcutaneous challenge (7 LD₅₀). Adoptive transfer experiments indicated that a minimum of 3×10⁷ six-day immune spleen cells were also able to protect recipient mice against intraperitoneal challenge with 10 LD₅₀ of SFV. Treatment of these donor cells with cytotoxic antisera and complement revealed both T- and B-lymphocytes were required for optimum adoptive immunity. Surviving recipients of either immune serum or immune spleen cells developed significantly less neutralizing antibodies than control mice. The lower antibody titres in protected mice might be related to either immune serum or immune spleen cell mediated restriction of virus replication; meaning a reduced antigenic stimulus in these mice compared to control mice.