Which ligand interactions correctly match the following cell pairs: APC with CD4+ T cell, virus-infected cell with CD8+ T cell, and CD4+ T cell with B cell?

Which ligand interactions correctly match the following cell pairs: APC with CD4+ T cell, virus-infected cell with CD8+ T cell, and CD4+ T cell with B cell?

• A. APC–CD4+ T cell: MHC-II with TCR, CD4; B7 with CD28; Virus–CD8+ T cell: MHC-I with TCR, CD8; CD4+ T cell–B cell: CD40L with CD40
• B. APC–CD4+ T cell: MHC-II with TCR, CD4; CD40 with CD28; Virus–CD8+ T cell: MHC-I with TCR, CD8; CD4+ T cell–B cell: CD40 with CD40L
• C. APC–CD4+ T cell: MHC-II with TCR, CD4; B7 with CD28; Virus–CD8+ T cell: MHC-I with TCR, CD8; CD4+ T cell–B cell: CD40L with CD40
• D. APC–CD4+ T cell: MHC-I with TCR, CD8; B7 with CD28; Virus–CD8+ T cell: MHC-II with TCR, CD4; CD4+ T cell–B cell: CD40 with CD40L

Answer: (A)

Two signals drive T cell activation. First, the T cell receptor recognizes peptide presented by the appropriate MHC class with the right co-receptor (CD4 for MHC II, CD8 for MHC I). Second, a co-stimulatory interaction boosts the signal (B7 on the antigen-presenting cell binds CD28 on the T cell). For helper T–B cell collaboration, CD40L on the T cell engages CD40 on the B cell to promote activation, isotype switching, and germinal center development. APCs present to CD4+ T cells via MHC class II, with the CD4 co-receptor helping stabilize signaling, and provide co-stimulation through B7 binding CD28. Virus-infected cells present to CD8+ T cells via MHC class I with CD8, delivering the TCR signal and CD8 co-receptor participation. CD4+ T cells then provide help to B cells through CD40L binding to CD40 on B cells, driving their activation and antibody responses. So the combination that matches these interactions is: MHC II with TCR and CD4 for APC–CD4+ T cell, B7 with CD28 for co-stimulation; MHC I with TCR and CD8 for virus–CD8+ T cell; CD40L with CD40 for CD4+ T cell–B cell help.

Two signals drive T cell activation. First, the T cell receptor recognizes peptide presented by the appropriate MHC class with the right co-receptor (CD4 for MHC II, CD8 for MHC I). Second, a co-stimulatory interaction boosts the signal (B7 on the antigen-presenting cell binds CD28 on the T cell). For helper T–B cell collaboration, CD40L on the T cell engages CD40 on the B cell to promote activation, isotype switching, and germinal center development.

APCs present to CD4+ T cells via MHC class II, with the CD4 co-receptor helping stabilize signaling, and provide co-stimulation through B7 binding CD28. Virus-infected cells present to CD8+ T cells via MHC class I with CD8, delivering the TCR signal and CD8 co-receptor participation. CD4+ T cells then provide help to B cells through CD40L binding to CD40 on B cells, driving their activation and antibody responses.

So the combination that matches these interactions is: MHC II with TCR and CD4 for APC–CD4+ T cell, B7 with CD28 for co-stimulation; MHC I with TCR and CD8 for virus–CD8+ T cell; CD40L with CD40 for CD4+ T cell–B cell help.