A pathway of costimulation that prevents anergy in CD28- T cells: B7-independent costimulation of CD1-restricted T cells.
A class of molecules that is expressed on antigen presenting cells, exemplified by CD80 (B7), has been found to provide a necessary costimulatory signal for T cell activation and proliferation. CD28 and CTLA4 are the B7 counterreceptors and are expressed on the majority of human CD4+ T cells and many CD8+ T cells. The signal these molecules mediate is distinguished from other costimulatory signals by the finding that T cell recognition of antigen results in a prolonged state of T cell unresponsiveness or anergy, unless these costimulatory molecules are engaged. However, nearly half of the CD8+ and CD4-CD8- T cells lack CD28, and the costimulatory signals required for the activation of such cells are unknown. To understand the pathways of activation used by CD28- T cells, we have examined the costimulatory requirements of antigen-specific CD4-CD8- TCR(+)-alpha/beta circulating T cells that lack the expression of CD28. We have characterized two T cell lines, DN1 and DN6, that recognize a mycobacterial antigen, and are restricted not by major histocompatibility complex class I or II, but by CD1b or CD1c, two members of a family of major histocompatibility complex-related molecules that have been recently implicated in a distinct pathway for antigen presentation. Comparison of antigen-specific cytolytic responses of the DN1 and DN6 T cell lines against antigen-pulsed CD1+ monocytes or CD1+ B lymphoblastoid cell lines (B-LCL) demonstrated that these T cells recognized antigen presented by both types of cells. However, T cell proliferation occurred only when antigen was presented by CD1+ monocytes, indicating that the CD1+ monocytes expressed a costimulatory molecule that the B-LCL transfectants lacked. This hypothesis was confirmed by demonstrating that the T cells became anergic when incubated with the CD1(+)-transfected B-LCL in the presence of antigen, but not in the absence of antigen. The required costimulatory signal occurred by a CD28-independent mechanism since both the CD1+ monocytes and CD1+ B-LCL transfectants expressed B7-1 and B7-2, and DN1 and DN6 lacked surface expression of CD28. We propose that these data define a previously unrecognized pathway of costimulation for T cells distinct from that involving CD28 and its counterreceptors. We suggest that this B7-independent pathway plays a crucial role in the activation and maintenance of tolerance of at least a subset of CD28- T cells.
D000938:Antigen-Presenting Cells / Q000276:immunology*; D018949:Antigens, CD1 / Q000502:physiology*; D001402:B-Lymphocytes / Q000276:immunology*; D018122:B7-1 Antigen / Q000502:physiology*; D018106:CD28 Antigens / Q000502:physiology*; D002450:Cell Communication; D002478:Cells, Cultured; D017635:Clonal Anergy; D006801:Humans; D008213:Lymphocyte Activation; D009000:Monocytes / Q000276:immunology*; D016693:Receptors, Antigen, T-Cell, alpha-beta / Q000502:physiology; D016176:T-Lymphocyte Subsets / Q000276:immunology*
D018949:Antigens, CD1; D018122:B7-1 Antigen; D018106:CD28 Antigens; D016693:Receptors, Antigen, T-Cell, alpha-beta