Hijacking the T-cell communication network by the human T-cell leukemia/lymphoma virus type 1 (HTLV-1) p12 and p8 proteins

Abstract

The non-structural proteins encoded by the orf-I, II, III, and IV genes of the human T-cell leukemia/lymphoma virus type 1 (HTLV-1) genome, are critical for the modulation of cellular gene expression and T-cell proliferation, the escape from cytotoxic T-cells and natural killer cells, and virus expression. In here, we review the main functions of the HTLV-1 orf-I products. The 12kDa product from orf-I (p12) is proteolytically cleaved within the endoplasmic reticulum (ER) to generate the 8kDa protein (p8). At the steady state, both proteins are expressed at similar levels in transfected T-cells. The p12 protein remains in the ER and cis-Golgi, whereas the p8 protein traffics to the cell surface and is recruited to the immunological synapse. The p12 and the p8 proteins have seemingly opposite effects on T-cells; the ER resident p12, modulates T-cell activation and proliferation, whereas p8 induces T-cell anergy. The p8 protein also increases the formation of cellular conduits, is transferred to neighboring T-cells, and increases virus transmission. The requirement for HTLV-1 infectivity of orf-I is demonstrated by the loss of virus infectivity in macaques exposed to an engineered virus, whereby expression of orf-I was ablated. Altogether the current knowledge demonstrates that the concerted activity of p8 and p12 is essential for the persistence of virus infected cells in the host.

Figure 1. Amino acid sequence and domains within the p12 and p8 proteins

HTLV-1 p12 is a highly hydrophobic protein that contains an amino terminus noncanonical ER retention/retrieval motif (MLFRL), four putative proline-rich (PXXP) Src homology 3 (SH3)-binding domains, two putative leucine zipper (LZ) motifs, an adaptin motif, and a calcinuerin binding motif. The arrows indicate the two cleavage sites between amino acid positions 9 and 10; 29 and 30, respectively.

Figure 2. p12 induces the dimerization of the ILR β and γ chains and increases STAT5 activation

In the ER, p12 binds to both the IL-2 the β and γ_c chains and decreases the trafficking of the two IL-2R chains to the cell surface. p12 increases STAT5 phosphorylation and IL-2 gene transcription. This leads to the decreased requirement for IL-2 in HTLV-1-infected cells.

Figure 3. Opposing effects of the Orf-I products on NFAT activation

Upon phorbol myristate acetate (PMA) stimulation, which releases ER calcium-stores in a TCR-independent mechanism, p12 increases NFAT dephosphorylation. The dephosphorylated NFAT is translocated into the nucleus, where it activates IL-2 production and favors T-cell growth (left part of the Figure). The Orf-I product(s), however, also bind(s) to calcineurin, competes with calcineurin for binding to NFAT and decrease T-cell activation (right part of the Figure).

Figure 4. p12 binds to and reroutes the MHC I Heavy chain for degradation to the proteasome

The p12 protein interacts with the MHC I heavy chain (Hc) in the ER and prevents its association with the beta-2-microglobulin (β₂m). The lack of association of MHC I HC with the β₂m causes its retrotranslocation into the cytosol and degradation by the proteasome. This ultimately leads to a decreased MHC I cell surface expression.

Figure 5. p8 localizes at the immunological synapse and decreases proximal T-cell signaling

p8 decreases T-cell activation after TCR-stimulation through its interaction with LAT, which causes the decrease in phosphorylation of LAT, PLC-γ1, Vav, and Lck and decreased activity of NFAT.

Figure 6. p8 enhances viral spread by increasing cell-cell contact

The p8 protein increases LFA-1 clustering at the cell surface leading to increased cell-cell contact. In addition, p8 stimulates the formation of cellular conduits. These modes of cell contact lead to increased p8 transfer to neighboring cells and ultimately viral transmission.