T cells begin to express both CD4 and CD8 in the thymic cortex.
In the thymus, immature T-cells begin as double-negative (lacking both CD4 and CD8). Following T-cell receptor (TCR) gene rearrangements in the thymic cortex, the immature T-cells begin expressing both CD4 and CD8. At this stage, the T-cell is called a double-positive immature T-cell. Immature double-positive T-cells transition to single-positive CD4 or CD8 T-cells following positive selection. This is also a process that occurs in the thymic cortex and involves cortical epithelial cells. Negative selection is accomplished in the thymic medulla and is responsible for the apoptosis of self-reactive T cells.
Cooper et al. discuss primary immunodeficiency. They note that T-cell and mixed B/T cell disorders typically present before 6 months of age with failure to thrive, oral thrush, graft-versus-host-disease from maternal lymphocytes, and excessive diarrhea. Various opportunistic infections such as CMV, EBV, varicella, enterovirus, Candida, and Pneumocystis carinii are implicated.
Alam et al. discuss positive selection. In the cortex, if a thymocyte is able to recognize an MHC molecule, it is rescued from apoptosis (positive selection). In the medulla, T cells expressing TCRs with high affinity for self antigen are destroyed. How a similar interaction by a TCR can result in two separate outcomes is a matter of current study.
Illustration A summarizes the progression of T-cell development within the thymus.
Answer 1: T-cell progenitors in the bone marrow are double (CD4 and CD8) negative until they reach the thymic cortex and undergo TCR recombination.
Answer 2 & 5: B-cells and macrophages do not express CD4 or CD8 receptors.
Answer 4: T-cells in the thymic medulla are either CD4+ or CD8+ but not both.
Cooper MA, Pommering TL, Korányi K. Primary immunodeficiencies. Am Fam Physician. 2003 Nov 15;68(10):2001-8. Review.
PMID:14655810 (Link to Abstract)
Alam SM, Travers PJ, Wung JL, Nasholds W, Redpath S, Jameson SC, Gascoigne NR. T-cell-receptor affinity and thymocyte positive selection. Nature. 1996 Jun 13;381(6583)
PMID:8637599 (Link to Abstract)