DISCUSSION
Prior to the discovery of hybridoma technology to generate monoclonal antibodies, suppressor T cell activity in humans was first shown by Waldmann et al (1) and Siegal et al (2). The activity of suppressor T cells was increased in patients with primary immunodeficiency, and appeared to play a role in decreased immunoglobulin production by B cells. Shou et al (24) and Gupta et al (4) reported generation of suppressor T cells activity following activation with concanavalin A, which was altered in patients with primary immunodeficiencies (25). Schwartz et al (26) demonstrated that concanavalin A (Con A)-activated PBMC suppressed differentiation of B cells to immunoglobulin synthesizing and secreting plasma cells. We showed that Con A-activated purified T cells suppressed immunoglobulin production by B cells and Con A activation of T cells was associated with increase in Tγ cells that were considered suppressor T cells (4). Following availability of monoclonal antibodies to define T cell subsets, Strelkauskas et al (3) reported natural suppressor activity in OKT8+ (CD8+) T cells, whereas helper activity was in OKT4+ (CD4+) T cells. Damle and Gupta (6), and Reinherz et al (27) reported that Con-A induced suppressor activity of T cells was in CD8+ T cells. Now more than 3 decades later interest in CD8 Treg cells has revived. In a number of experimental models and human autoimmune diseases, and cancer, its role in the regulation of immune response has been established. Shi et al [19] reported that human CD8+CD183+ T cells have same function as murine CD8+CD122+ Treg, and suppression of CD8+ T cell function is mediated by IL-10.
A number of studies have demonstrated that CD8+ Treg cells induce the expression of inhibitory receptor on monocytes and dendritic cells, suppress the proliferation of effector and memory CD4+ and T cells, and downregulate IFN-γ production by CD8+ T cells (23, 28, 29). However, regulatory effects of CD8 Treg on B cell functions have not been evaluated.
Investigators have used different surface molecules including LAP-1, CD39, PD-1, and CTLA-4 to define CD8 Treg to study their functions (20-23). In the present study, to further defined CD8 Treg, we utilized all these molecules and examined their expression on FoxP3 CD8+ T cells. Our data show that FoxP3+CD8+ T cells preferentially express CD25, CD278, CD183, CTLA-4, and LAP-1. LAP-1 is an intracellular molecule. Therefore, we utilized sorted CD8+CD25highCD183+CD278+ Treg to compare their regulatory effect from CD8+CCR7+CD183+CD45RA- Treg cells.
In the present study, we have demonstrated that CD183+ CCCR7+CD45RA- CD8+ T reg significantly inhibited B cell proliferation and suppressed IgM and IgG secretion by B cells only at B; CD8 Treg ratio of 1:1; however, they did not significantly suppress IgA production. In contrast, CD25highCD278+CD183+CD8+ Treg significantly inhibited B cell proliferation at B cell: CD8 Treg ratio of 1:1 and 1:1/2, and significantly suppressed all immunoglobulin isotypes, IgM, IgG, and IgA production at all B: CD8 Treg ratios. The suppression of Immunoglobulin production by B cells would be consistent with inhibition of plasmablasts by CD8 Treg (30). Furthermore, CD8 Treg, define by both sets of markers, suppressed IgM production to a greater extent than IgG and IgA. We have reported increased CD8 Treg cells in patients with primary selective IgM deficiency (31) that may be one of the mechanisms of selective IgM deficiency. CD8 Treg have been shown to regulate T cell responses by the production of IL-10 (19, 23). A number of additional mechanisms have been reported for CD8 Treg mediated suppression of T cell responses. Chen et al (21) reported that CD8+LAP-1+FoxP3+ CD8 Treg suppress EAE that is dependent upon both TGFβ and IFNγ. However, we did not observed significantly increased expression of LAP-1 in FOXP3+ CD8 T cells as compared to FoxP3- CD8 T cells. Several investigators have reported cell-to cell contact is required for CD8 Treg-mediated suppression (23, 32, 33). Cosimi et al (23) reported abrogation of CD8 Treg-mediated suppression of T cell responses by anti-CTLA-4 and TGFβ antibodies. Cai et al (33) reported CD8 Treg-mediated suppression via IL-10, TGFβ and CTLA-4. The mechanism (s) of CD8 Treg-mediated suppression of B cell proliferation and immunoglobulin production remains to be investigated.
In summary, CD8 Treg inhibit B cell proliferation and differentiation into immunoglobulin producing cells. CD25highCD278+CD183+CD8+ T reg were more effective in regulating B cell responses as compared to CD183+ CCCR7+CD45RA-+ CD8+ Treg. CD8 Treg-mediated suppression of antibody production may be one of the mechanisms of its role in autoimmune diseases, and in antibody deficiency disorders.