Sequence Changes At the V-D Junction of the V(H)1 Heavy Chain of Anti-Phosphocholine Antibodies Alter Binding to and Protection Against Streptococcus Pneumoniae

X-linked immune deficient (Xid) mice fail to produce anti-phosphocholine (PC) antibodies even after immunization with Streptococcus pneumoniae. Consequently, Xid mice are extremely susceptible to infection with S. pneumoniae. PC-specific a cells appear to undergo clonal deletion in Xid mice; however, a new thymus-dependent form of PC, 6-(O-phosphocholine)hydroxyhexanoate (EPC), can rescue PC-specific B cells from the bone marrow presumably by providing T cell help before clonal deletion. Analysis of PC-specific IgG hybridomas from Xid mice revealed utilization of several V-D junctional variants of the V(H)1 gene segment rearranged to different D and J(H) gene segments. The majority of Xid anti-PC antibodies exhibit an Asp --> Gly95H replacement at the V-D junction. These Gly95H V(H)1 variants associate with kappa 1C L chains to produce anti-PC antibodies that: (i) have low relative affinity for PC, (ii) are heteroclitic for nitrophenylphosphocholine and (iii) fail to bind to or provide protection against S. pneumoniae. Single prototypic V-D variants of the T15 idiotype (Asp95H), M603 idiotype (Asn95H) and M167 idiotype (Asp95H-Ala96H) were also induced in Xid mice. The M603-like and M167-like antibodies bound to and protected against S. pneumoniae even though they exhibited K(a)s for PC which were lower than T15 idiotype(+) antibodies. These data demonstrate that small changes in the V-D junctional sequence of the T15 (V(H)1) heavy chain alter L chain usage and the structure of the PC binding site so that the PC expressed on S. pneumoniae is no longer recognized. [References: 54]

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