Endemic Lassa fever in Liberia. IV. Selection of optimally effective plasma for treatment by passive immunization.

The efficacy of passive immunization for treatment of Lassa Fever (LF) is believed to depend on the titre of the neutralizing antibody infused. For the purpose of identifying optimal donors of LV-immune plasma, a population of LF-convalescent patients in Liberia was tested for prevalence of neutralizing antibody. Minimally protective titres, expressed as a log10 neutralization index, (LNI), were established in animal models as LNI greater than 2. LNI titres for 26 donors, tested eight or more months after illness, were modest: 16 titred 1 less than LNI less than 2, 4 titred 2 greater than LNI less than 3, and only 4 titred LNI greater than 3. Sequentially obtained plasma from six donors indicated that the LNI response was delayed relative to the indirect fluorescent antibody (IFA) response, that high titres (LNI greater than 3) occurred only after seven months and in only two of six patients. Most of the unselected LV-immune plasma will require concentration to therapeutically useful LNI titres. In a passive immunization experiment, guinea-pigs were protected by a late convalescent plasma (LNI = 4.8, IFA = 320) but not by an early plasma, (LNI = 0.6, IFA = 640), thus supporting the selection of immune plasma on the basis of the LNI. Cross serological testing with LV strains and convalescent plasma from patients in Sierra Leone, Liberia and Nigeria suggested that these LV strains were indistinguishable by cross-IFA, but were readily distinguishable by cross neutralization tests. Geographical matching of LV and plasma origins may thus be a factor in selection of optimal plasma for passive immunization of Lassa fever.