It has been well-established that the major histocompatibility complex (MHC) plays a pivotal role in the control of immune responses. The MHC of humans is denoted HLA in humans, and H-2 in mice. MHC encoded cell surface antigens consist of polymorphic, membrane-bound glycoproteins that are expressed on virtually every cell. These molecules play a central role in the control of antigen presentation by presenting peptide fragments at the surface of cells, and by presenting processed peptides to T cells. T cells are thought to be activated by either a peptide-MHC complex or an altered peptide ligand that binds to MHC. The former pathway is referred to as the “peptide-MHC complex” pathway, and the latter pathway is referred to as the “altered peptide ligand” pathway. T cells do not generally recognize free peptides, but peptides associated with MHC molecules. Exceptions to this rule are certain viruses (e.g. HIV-1) where peptides are generated in the cytosol and are transported into the endoplasmic reticulum or MHC class I molecules in the absence of the conventional peptide editing machinery. The MHC class I pathway is not always T cell dependent, as illustrated by CD8+ T cells that are activated in the absence of MHC class I molecules (MHC-I) by a process known as “cross-presentation”. In the “peptide-MHC complex” pathway, T cells recognize and can bind directly to peptides presented by MHC. Complexes of MHC class I molecules and peptides are generated in the cytosol by the proteasome complex. The MHC class I peptide complexes then travel to the cell membrane, either from the site of protein synthesis or from other sites within the cell, and there they are expressed on the cell surface. Peptides are generally produced from cellular proteins in a 20-30 amino acid length by a process called proteasomal degradation. Proteins are produced in the cytosol, and after the protein is fully synthesized, proteasomes release peptides as short as three amino acids in length. This peptide mixture is then transported into the endoplasmic reticulum by TAP molecules (transporters associated with antigen presentation), where it is “trimmed” to eight or nine amino acids in length. Then, the peptides are transported to the endoplasmic reticulum by the TAP complexes and loaded onto newly synthesized MHC class I molecules that are assembled together with chaperone proteins. The MHC-I/peptide complex is then transported to the cell membrane where the peptide is presented to the T cell receptors. The “altered peptide ligand” pathway is less well-understood. In this pathway, MHC molecules do not bind peptides, but instead bind non-peptidic fragments of proteins, which are generated through the cleavage action of an enzyme complex of the ubiquitin pathway. These non-peptidic fragments of proteins are then presented on the cell surface and can be recognized by CD8+ T cells. The MHC class I pathway is highly regulated and involves a variety of positive and negative selective steps. Although the MHC class I pathway is responsible for the elimination of virally infected cells, it also has the potential to induce autoimmune diseases if it malfunctions. For example, some viruses such as the retrovirus HTLV-1 down-regulate the genes that encode the endogenous proteins (MHC-I) and MHC-II in infected cells. In contrast, other viruses, such as the hepatitis B and C viruses, induce over-expression of MHC molecules (MHC class I and II) to increase the susceptibility of infected cells to CD8+ T cell recognition. Cytotoxic CD8+ T cells (CTLs) are able to recognize and eliminate abnormal cells in the body such as cancer cells. The target antigens recognized by CTLs are intracellular antigens, or antigens that are expressed on the cell surface in the form of a complex with MHC molecules. Intracellularly, viruses can express proteins which are degraded by proteasomes into peptides which are then transported into the endoplasmic reticulum by the TAP molecules (transporters associated with antigen presentation), where they are loaded onto newly synthesized MHC class I molecules. The complex is then transported to the cell membrane where the peptide is presented to the CTLs. Examples of intracellular viral proteins that are processed in this manner include HIV-1 Gag protein, and the Epstein Barr virus LMP proteins. CTLs are able to recognize this class of antigens because they contain fragments of viral proteins. Virus-specific CTLs recognize and kill their target cells via the TCR/CD3 complex on their surface. A common problem with virus-specific CTLs is that they often recognize and kill virus infected cells, and also normal uninfected host cells. This can lead to tissue destruction, the disease condition itself or exacerbate an already existing infection. This situation can be particularly problematic when the virus is a human retrovirus such as HIV-1, as elimination of infected cells may result in the destruction of surrounding uninfected host cells that may be necessary for a patient's health. Therefore, it is important to limit the response of CTLs to virus infected cells. Several reports have documented that the T cell response is directed towards epitopes found only in the virus strain the immune system is exposed to. This has been reported in CD8+ T cell responses in animal models to the simian immunodeficiency virus and the murine retrovirus called M-MuLV. See Picker et al. Science 1997, Vol. 276, 111-113; Zahn et al. J. Virology 1995, 6950-6; and Weckerle et al. Intl. Immunol. 1992, Vol. 4, 861-869). The phenomenon is particularly interesting since CTLs are thought to eliminate infected cells through the recognition of viral proteins by their TCRs. Thus, to have an effective CTL response, the virus must express viral proteins that are processed properly for MHC binding and then transported to the cell surface in a form that is recognized by the CTL. These viral proteins must also be appropriately presented to T cells in the context of MHC class I. In order to obtain a productive infection, viruses need to express the appropriate viral proteins to “fool” CTLs. However, viral pathogens have evolved different methods to escape CTL recognition. Viruses can, for example, infect cells without undergoing the normal cellular pathways of protein processing and/or loading on MHC class I molecules, as is the case with HTLV-1. Alternatively, viruses have been reported to “evade” CTLs by expressing low levels of viral proteins or proteins that bind well to MHC but do not allow for proper MHC class I molecule loading. Human immunodeficiency virus type 1 (HIV-1) is an enveloped retrovirus that infects CD4+ lymphocytes and causes the acquired immunodeficiency syndrome (AIDS). The viral particle contains three major structural proteins: a nucleocapsid protein (NC) and two envelope glycoproteins, the surface protein gp120 and the transmembrane protein gp41. The envelope glycoprotein of HIV-1, gp120, binds to cell surface CD4 receptors and mediates fusion of the virus with the target cell. The transmembrane protein gp41 attaches to the gp120 protein of the virion. Thus, gp120 and gp41 are processed through the cellular MHC class I pathway. HIV-1 is able to survive and replicate in CD4+ T cells in part because it evades the CD8+ T cell response by inhibiting antigen processing. The envelope glycoprotein of HIV-1 is thought to play a critical role in viral infectivity, pathogenesis, and replication in the human host. HIV-1 envelope glycoprotein (gp160) contains a precursor polypeptide with a molecular weight of 160 kilodaltons (kDa), which is proteolytically cleaved by host cell proteases to yield two glycoproteins (gp120 and gp41) with molecular weights of approximately 120 kDa and 41 kDa, respectively. See, e.g., Malim et al. J. Clin. Microbiol. 1988, 26, pp. 600-607, and Goudsmit et al. J. Virol. 1988, 62, pp. 2350-2360. Recent studies suggest that the N-terminal extracellular domain of the gp41 protein of HIV-1 is primarily responsible for the antigenic determinants, and, therefore, it is the major immunogen in gp41. See Stamatatos et al. J. Gen. Virol. 1992, 73, pp. 3391-3398. In addition to host cellular factors, several envelope genes of HIV-1 have been identified as important regulators of viral pathogenesis. See, e.g., Matsuoka et al. Science 1990, 248, pp. 853-856; Malim et al. J. Virol. 1990, 64, pp. 4095-4100; Yarchoan et al. Nature 1989, 339, pp. 426-430; and Nunberg et al. Nature 1988, 331, pp. 453-456. For example, the envelope transmembrane glycoprotein gp41 of HIV-1 is one of the key regulators of membrane fusion of HIV-1. See, e.g., Feng et al. Nature 1995, 377, pp. 467-471. The transmembrane protein gp41 is cleaved to produce smaller functional proteins, the N-terminal ectodomain (gp41) and the C-terminal ectodomain (gp36). The cleavage of gp41 generates two components of gp41 that function in the entry of HIV-1 into target cells: the ectodomain of gp41 (gp41e) and the truncated portion of gp41 (gp41s). The C-terminal ectodomain of gp41 (gp41s) is a non-covalent homodimer in which the gp41s homodimer bridges the viral envelope and target cell membranes