Increased Percentage of CD8⁺CD28^- Regulatory T Cells With Fingolimod Therapy in Multiple Sclerosis

Fingolimod, an oral therapy for MS, decreases expression of membrane S1P1 receptors on CD4⁺ memory cells, causing their retention and deactivation in lymph nodes. We determined fingolimod effects on the number and proportion of potentially CNS-damaging CD8⁺CD28⁺ cytolytic T lymphocyte cells (CTLs) and on MS-depleted and dysfunctional CD8⁺CD28^- anti-inflammatory suppressor/regulatory T cells (Treg) and on CD8⁺ T-cell expression of the CD69 activation/lymph node retention protein in MS.

CD8, CD28, CD4, and CD69 expression on peripheral blood mononuclear cells was measured with flow cytometry. In vitro concanavalin A (ConA) activation of T cells, including CD8⁺CD28^- cells, was used to mimic inflammation.

Fifty-nine patients with MS, 35 therapy-naive (16 clinically stable; 19 exacerbating) and 24 fingolimod-treated (19 clinically stable; 5 exacerbating), and 26 matched healthy controls (HCs) were compared. In therapy-naive patients, the CD8⁺ Treg percent of total lymphocytes was only 1/4 of HC levels. In fingolimod-treated patients, however, CD8⁺ Treg percentages rose to 2.5-fold higher than in HC and 10-fold higher than in therapy-naive MS. With fingolimod therapy, in contrast, CD8⁺ CTL levels were less than half of levels in HCs and therapy-naive patients. In HCs and all MS, activation with ConA strongly induced CD69 expression on CD4⁺ cells and induced 3-fold higher CD69 levels on CD8⁺ CTL than on CD8⁺ Treg. Fingolimod and analogs in vitro did not modify lymphocyte CD69 expression. Lower levels of CD69 on CD8⁺ Treg than on CTL may allow easier Treg egress from lymph nodes and enhance control of peripheral inflammation. In vitro activation reduced the already low CD8⁺ Treg population in therapy-naive MS, but only slightly altered Treg levels in fingolimod-treated MS.

Fingolimod therapy markedly increases the percentage of CD8⁺ Treg in MS, reversing the low CD8⁺ Treg:CTL ratio seen in untreated MS. The increase in immune regulatory cells has potential therapeutic benefit in MS. Activation in vitro depletes CD8⁺CD28⁺CTL in patients with MS; the loss is more pronounced in older patients with MS. This suggests that inflammation can disrupt the tenuous immune regulation in MS, especially in older patients.