Alterations in immune cell phenotype and function after experimental spinal cord injury

Traumatic injury to the spinal cord initiates a cascade of inflammatory-mediated injury and repair processes within the nervous system. In parallel, spinal injury could influence peripheral mechanisms of host defense (e.g., wound healing, antibody production) by altering lymphocyte phenotype and function. The goal of this study was to evaluate the physiological impact of spinal contusion injury on phenotypic and functional indices of lymphocyte activation. A flow cytometric time-course analysis of lymphocytes isolated from lymph node and spleen revealed an increase in CD4+ and a decrease in CD8+ lymphocytes during the first week post injury. The functional potential of lymphocytes was also evaluated based on their ability to proliferate in the presence of a biologically relevant antigen (myelin basic protein, MBP) or a lymphocyte mitogen. The data revealed increased proliferation to MBP by 3 days postinjury in lymphocytes isolated from lymph node but not spleen. By 1 week postinjury, increased proliferation to mitogen was noted in both the lymph node and the spleen suggesting a general increase in lymphocyte reactivity during this time interval. Circulating corticosterone (CORT), an endogenous glucocorticoid with significant effects on lymphocyte phenotype and function, was elevated within 24 h after spinal cord injury (SCI) and remained above control levels throughout the duration of our studies (up to 1 month postinjury). The present data suggest injury-associated changes in immune cell phenotype and function paralleled by the activation of the hypothalamic-pituitary-adrenal (HPA) axis.