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The homing of lymphocytes to peripheral lymph nodes is initiated by an adhesive interaction between L-selectin on lymphocytes and PNAd, a set of sialomucins that are constitutively displayed on high endothelial venules of lymph nodes. PNAd is defined by monoclonal antibody MECA-79 that recognizes a sulfated oligosaccharide carried by the sialomucins. This epitope overlaps with 6-sulfo sialyl Lewis x, a recognition determinant for L-selectin. Previous work has shown that administration of a L-selectin monoclonal antibody blocks both late-phase airway responses and airway hyperresponsiveness in a sheep model of asthma. We show here that airway-associated lymphoid collections from lungs of allergic sheep exhibited PNAd(+) venules as detected by immunostaining with MECA-79. The same vessels also expressed a GlcNAc-6-O-sulfotransferase known as HEC-GlcNAc6ST, which is known to contribute to the formation of the MECA-79 epitope in high endothelial venules of mouse lymph nodes. Intravenous administration of MECA-79 to allergic sheep significantly blunted both the late-phase airway response and airway hyperresponsiveness induced by airway allergen challenge. Furthermore, MECA-79 inhibited the accumulation of all classes of leukocytes in bronchoalveolar lavage fluid. These findings represent the first demonstration that targeting of PNAd has therapeutic efficacy in an inflammatory disease.