CBA/N mice, which express the X-linked immunodeficiency gene xid, are susceptible to Salmonella typhimurium. The basis for this susceptibility is currently unknown. However, previous studies (10) from this laboratory have provided evidence that susceptibility may be due to a defective anti-S. typhimurium antibody response. In that report we hypothesized that the defective antibody response may be a reflection of an altered S. typhimurium-specific B cell repertoire. In the studies described here, we have investigated this hypothesis using a modification of the in vitro splenic focus system. The frequency and characteristics of salmonella-specific B cells in normal, innately resistant, CBA/Ca mice have been compared with those of salmonella-susceptible, anti-S. typhimurium antibody-defective CBA/N mice. The results show that CBA/N mice express no primary or secondary S. typhimurium-specific B cell precursors after stimulation with an acetone-killed and dried (AKD) preparation of S. typhimurium strain TML. However, after three immunizations, the CBA/N tertiary frequency of 15.4 per 10(6) splenic B cells was similar to the primary precursor frequency in immunologically normal CBA/Ca mice, but 23-fold lower than the tertiary precursor frequency in CBA/Ca control mice. Moreover, CBA/N mice had an altered isotype distribution pattern after stimulation with AKD-TML. Greater than 70% of the tertiary CBA/N TML-specific B cells secreted IgG2, in contrast to either nonimmune or primed control mice. In addition, 80% of the CBA/N TML-specific B cells secreted only a single isotype, whereas the majority of B cells from primed normal mice secreted multiple isotypes. Fine specificity analysis of the TML-specific B cells indicated that the array of antigenic determinants to which CBA/N B cells could respond was restricted. Although the majority of primed CBA/Ca and primed CBA/N B cells were specific for LPS, the fine specificity pattern exhibited by CBA/N B cells was similar to that observed in unprimed normal mice, i.e., the vast majority were specific for the O antigen region of the LPS molecule. In contrast, a major portion of the LPS-specific B cells in primed CBA/Ca mice were directed against the KDO/lipid A region of the LPS molecule. Therefore, it appears that CBA/N mice lack or are unable to stimulate the B cell subset that predominates in primed, normal mice. Taken together, these studies indicate that the basis for susceptibility of CBA/N mice to S. typhimurium is multifactorial and suggests that the inability of some animals to respond to some infectious agents may be related to holes in their B cell repertoire.

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