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3 Gene-Disrupted Mice Selectively Deficient in the Dominant IgG Subclass Made to Bacterial Polysaccharides Undergo Normal Isotype Switching After Immunization with Polysaccharide-Protein Conjugate Vaccines1



Departments of
*
Pediatrics and
Genetics, and
Institute of Pathology, Case Western Reserve University School of Medicine, Cleveland, OH 44106
Bacterial polysaccharides (PS) are T-independent type 2 Ags that
elicit restricted Ab responses of IgM and IgG3 in mice and IgM and
predominantly IgG2 in humans. Immunodeficiency in the dominant IgG
subclass made to PS is associated with chronic sinus and pulmonary
infections with PS-encapsulated bacteria. To elucidate the biologic
role of the dominant IgG subclass in the immune response to PS and to
make an animal model of human IgG subclass deficiency, we generated
mice with a targeted disruption of the exon encoding the CH1 domain of
the
3 heavy-chain constant region gene. Homozygotes had no
detectable serum IgG3, and their splenocytes did not produce IgG3 after
LPS stimulation. IgG3-/- mice immunized with PS from
Pseudomonas aeruginosa LPS O-side chain or
Streptococcus pneumoniae type 19F capsule did not
produce any IgG3 anti-PS Abs, in contrast to wild-type mice in
which IgG3 was the major IgG subclass. Immunizing both wild-type and
IgG3-/- mice with 19F PS-protein conjugate elicited IgG1
Abs. We conclude that IgG3-/- mice have a selective
deficiency in the dominant murine IgG subclass made to T-independent
type 2 Ags and may be a useful animal model of IgG subclass deficiency.
In addition, we show that the anti-PS Ab class switching to IgG1
that occurs when mice are immunized with a PS-protein conjugate vaccine
does not require sequential Ig expression or an intact, upstream
3
heavy-chain gene.
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