How many factor antisera can be found for serogroup 6 Streptococcus pneumoniae?

We read the article of Jacobs et al. (3) with interest. The authors reported they could identify serotype 6C Streptococcus pneumoniae using factor 6d, which was developed by Statens Serum Institut, Copenhagen, Denmark. We immunized a rabbit with one serotype 6C isolate identified with a monoclonal antibody (MAb6C) in the U.S. lab in which the new type was found. Based on absorption with serotypes 6A and 6B and the Quellung test (6), we got the specific antiserum for MAb6C after absorption with 6A and 6B. The antiserum identified 10 serotype MAb6C isolates among 91 traditional serotype 6A isolates, just like the multibead assay. We concluded that serotype 6C could be set up in the Denmark typing system because it could have a new factor antigen, 6X1 (5). More importantly, it must have a new antigenic formula, 6a, 6b, 6X1, which showed it must be a new serotype (2, 5). But we found the antiserum was not specific for serotype 6C in further study. The antiserum could be used to distinguish new type 6D from traditional 6B isolates. Six such isolates were identified among 111 isolates typed as 6B before (unpublished data). Type 6D was then confirmed using a PCR method that was described previously (4). The antiserum should be specific for serotypes 6C and 6D, and it was reported that the two serotypes have replacements similar to those in the capsular genes of serotype 6A and 6B (1). It was not unexpected that the two new types have the same antigen. We named the new factor antiserum 6X1 instead of 6d in our publication (5) because we did not determine the antiserum with all known serotype antigens (7). Meanwhile, we found that factor 6b, after absorption completely by serotype 6C, was still against serotype 6A, which means factor 6b might be divided into two factors. The specific antigen for 6A had already been shown by the multibead assay, which showed that serotype 6A bound to both monoclonal antibodies 6A and 6A whereas 6C bound to only one monoclonal antibody, 6A (8). The multibead assay based on monoclonal antibodies is an effective method to identify the serotypes of S. pneumoniae, but it could cause discrepancy because the monoclonal antibodies were developed from mice, whereas the traditional diagnostic antisera were developed from rabbits. We recommend, therefore, that the names of serotypes based on monoclonal antibodies contain special added markers, for example, MAb6A, MAb6B or m6A, m6B, etc., instead of being simply 6A, 6B, and 6C.