In some cases, the function of a gene can be assessed by reducing, or even eliminating, the expression of that gene in specific cells. This can be accomplished by harnessing a system known as RNA interference, or RNAi, that is present in many eukaryotic cell types. When small double-stranded RNA molecules (referred to as small interfering RNAs, or siRNAs) are introduced into cells, the two RNA strands will be separated and one will bind to an enzyme complex known as RISC (RNA-induced silencing complex). The bound siRNA targets the RISC complex to the mRNA to which it has homology, leading either to translation arrest or to degradation of the mRNA, and thus to silencing of the gene (Fig. A.48). For cells that are not easily transfected with siRNA molecules directly, such as primary lymphocytes and myeloid cells, gene silencing can be implemented by using recombinant viruses. In this case, genes encoding small hairpin RNAs (shRNAs) are introduced into viral vectors that can be packaged into infectious viral particles. The shRNAs encode small RNAs that form a double-stranded hairpin structure; these hairpins are processed by enzymes in the cell to generate the siRNAs needed for gene silencing (see Fig. A.48). Since many primary hematopoietic cell types are readily transduced with recombinant viruses, such as retroviruses and lentiviruses, shRNAs can be effectively used to silence genes in these cell types.
Fig. A.48 Knockdown of gene expression using the RNAi pathway. Small double-stranded RNA molecules with homology to an mRNA transcript will target the mRNA for degradation or translation arrest. This pathway is initiated by expression of a short hairpin RNA (shRNA), which can be produced from an expression vector that is introduced into cells, or by the direct transfection of cells with small double-stranded RNA molecules called siRNA. shRNA molecules are processed by the enzyme Dicer to generate siRNA duplexes. The siRNA duplexes bind to the RISC complex, which separates the two RNA strands, retaining the noncoding strand of the siRNA. This noncoding strand targets the siRNA–RISC complex to the mRNA, leading to mRNA degradation or translation termination.
Compiled by Daniel DiToro, Carson Moseley, and Jeff Singer, University of Alabama at Birmingham. Data based on CD designations made at the 9th Workshop on Human Leukocyte Differentiation Antigens.
* May function as dimers Compiled by Robert Schreiber, Washington University School of Medicine, St Louis, and Daniel DiToro, Carson Moseley, and Jeff Singer, University of Alabama at Birmingham.
Chromosome locations are for humans. Chemokines for which there is no human homolog are listed with the mouse chromosome.
† ELR refers to the three amino acids that precede the first cysteine residue of the CXC motif. If these amino acids are Glu-Leu-Arg (i.e. ELR+), then the chemokine is chemotactic for neutrophils; if they are not (ELR–) then the chemokine is chemotactic for lymphocytes
Compiled by Joost Oppenheim, National Cancer Institute, NIH.
Emil von Behring (1854–1917) discovered antitoxin antibodies with Shibasaburo Kitasato.
Baruj Benacerraf (1920–2011) discovered immune response genes and collaborated in the first demonstration of MHC restriction.
Bruce Beutler (1957–) discovered the role of the Toll-like receptor in innate immunity in mice.
Jules Bordet (1870–1961) discovered complement as a heat-labile component in normal serum that would enhance the antimicrobial potency of specific antibodies.