A marked decrease in l‐selectin expression by leucocytes in infants with Bordetella pertussis infection: leucocytosis explained?

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Abstract
Objective: Infants with Bordetella pertussis infection (whooping cough) have an unexplained lymphocytosis and leucocytosis characterized by an increase in small lymphocytes with convoluted and cleaved nuclei. To characterize these cells immunophenotyping using multiparameter flow cytometry was performed on leucocytes from a group of 11 infants aged 3–6 months with proven pertussis and from uninfected control subjects. Methodology: The panel of monoclonal antibodies used to elucidate leucocyte subtypes included activation, adhesion, costimulatory, memory, T-helper (Th) 1 and Th2 markers. Results: Patients with pertussis showed an increase in absolute numbers of neutrophils, monocytes, T lymphocytes (both CD4 and CD8), B lymphocytes (including CD10+/CD19+ haematogones) and natural killer (NK) cells. All leucocyte subgroups showed a marked decrease in l-selectin (CD62L) expression. The expression of other adhesion molecules CD11a, CD44 and CD54 on all leucocyte subgroups was unchanged. Expression of costimulatory molecules, CD49D and CD28 on T cells and CD80 and CD86 on monocytes, was unchanged. Lymphocyte activation markers CD69, CD25 and HLA-DR were unchanged. There was an increase in CD45RA+/CD45RO+/CD4+ cells (activated) and CD62L–/CD45RO+/CD4+ cells (Th1-like) but no increase in CD7–/CD4+ T cells (Th2-like). Conclusions: l-Selectin expression mediates extravasation of leucocytes into tissues and is important for homing of peripheral blood lymphocytes to lymph nodes. The significant down-regulation of l-selectin on leucocytes in pertussis infection may prevent leucocyte migration to areas of infection and homing and adhesion of T and B cells to peripheral lymphoid tissues. The increase in lymphocytes with Th1 phenotype may be required for effective immune response to the infective organism. These data provide a possible explanation for the absolute leucocytosis observed in this disease.