Sialic acid acetylesterase (SIAE) is an enzyme that negatively regulates B lymphocyte antigen receptor signaling and is required for the maintenance of immunological tolerance in mice1, 2. Heterozygous loss-of-function germline rare variants and a homozygous defective polymorphic variant of SIAE were identified in 24/923 Caucasian subjects with relatively common autoimmune disorders and in 2/648 Caucasian controls. All heterozygous loss-of-function SIAE mutations tested were capable of functioning in a dominant negative manner. A homozygous secretion-defective polymorphic variant of SIAE was catalytically active, lacked the ability to function in a dominant negative manner, and was seen in 8 autoimmune subjects but in no control subjects. The Odds Ratio for inheriting defective SIAE alleles was 8.6 in all autoimmune subjects, 8.3 in subjects with rheumatoid arthritis, and 7.9 in subjects with type I diabetes. Functionally defective SIAE rare and polymorphic variants represent a strong genetic link to susceptibility in relatively common human autoimmune disorders.

A longitudinal study of diarrhea was carried out from May 1988 to April 1989 by household surveillance of 705 children less than 5 years old in rural Bangladesh. Stool samples were examined for enteric pathogens at the beginning of each diarrheal episode. For persistent episodes, stool examination was repeated on days 15-17 of the illness. For each case of persistent diarrhea, stool samples from age-matched acute diarrheal and healthy controls were examined. Compared with healthy controls, cases of diarrhea were associated with Shigella species (P = .07) and rotavirus (P less than .05). Diffusely adherent Escherichia coli (P less than .05) and cryptosporidia (P = .07) were the only enteropathogens associated with persistent diarrhea in comparison with acute diarrhea. No more than 15% of children had the same class of pathogen identified from stool on both days 1-3 and days 15-17, indicating that persistent infection was uncommon. However, a different enteropathogen was frequently found on days 15-17, suggesting that sequential infection may be a cause of persistent diarrhea.

Generation of Siae ⌬ 2/ ⌬ 2 mice Exon 2 of the Siae gene is unique to Lse . An engineered inframe deletion of exon 2 in a murine Lse complementary DNA resulted in a protein that lacked esterase activity ( Fig. 1 A ). Genomic deletion of exon 2 was achieved as described in the Materials and methods ( Fig. 1 B ). After germline transmission, homozygous mutant mice were generated and were found to be viable. Truncated exon 2 -defi cient Siae mRNA could be detected in KO mice (Fig. S3, available at http://www .jem.org/cgi/content/full/jem.20081399/DC1). Cytosolic esterase mRNA continues to be transcribed in these mutant mice. We refer to these KO animals as Siae ⌬ 2/ ⌬ 2 mice.Enhanced B lymphocyte antigen receptor signaling in Siae ⌬ 2/ ⌬ 2 mice Because Siae has the potential to remove 9-O -acetyl residues from ␣ 2 -6-linked sialic acid containing Siglec ligands, we predicted that B cells from mice lacking this esterase might exhibit enhanced BCR signaling similar to that noted in CD22-null mice ( 13 -16 ). Mice were fi rst bred into the C57BL/6 background for 10 generations. B cells from WT and Siae ⌬ 2/ ⌬ 2 mice were gated on, and the accumulation of cytoplasmic calcium after ligation of the BCR was analyzed using fl ow cytometry. As seen in Fig. 2 , BCR cross-linking resulted in an accelerated and enhanced calcium fl ux. A similar result was seen when purifi ed splenic B cells from mutant and WT mice were analyzed (Fig. S4, available at http://www.jem.org/cgi/content/ full/jem.20081399/DC1). These data suggested that in the absence of functional Siae, BCR signal strength is markedly enhanced and that this alteration in signal strength is an intrinsic property of mutant B lymphocytes.Defective CD22 signaling and hyperacetylation of ␣ 2 -6-linked sialic acid moieties on Siae mutant B cells The defect in Siae could result in the increased acetylation of ␣ 2 -6-linked sialic acid on N -glycans in B cells and, thus, attenuate the ability of glycoproteins on B cells to ligate CD22 and generate inhibitory signals. We sought to examine if there was a defect in CD22 signaling in Siae ⌬ 2/ ⌬ 2 mice. After BCR cross-linking, CD22 was isolated by immunoprecipitation, and immunoprecipitates were examined for CD22 tyrosine phosphorylation and for associated SHP-1 using Western blot assays. As seen in Fig. 3 (left, experiment 1; and right, experiment 2), tyrosine phosphorylation of CD22 after BCR signaling was reduced in Siae ⌬ 2/ ⌬ 2 mice in spite of similar levels of surface CD22 expression in WT and mutant mice ( It remained to be demonstrated whether a defect in Siae would result in enhanced 9-O-acetylation of ␣ 2 -6-linked sialic acid in Siae ⌬ 2/ ⌬ 2 mice. 9-O -acetylation of sialic acid has not