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Escape of HIV-1 from Cytotoxic T Lymphocytes In Vitro O. O. YANG*1, P. T. NGYUEN2, C. BRANDER2, S. A. KALAMS2, and B. D. WALKER2.
1Univ. of California at Los Angeles Med. Ctr.; and 2Massachusetts Gen. Hosp., Boston MHC class I-restricted, CD8+ cytotoxic T lymphocytes (CTL) are presumed to be an important component of antiviral immunity in HIV-1 infection. However, attempts to demonstrate that CTL exert immune pressure on HIV-1 in vivo have yielded mixed results. It is thus unclear whether lack of escape mutations in vivo might be due to ineffective CTL or whether the CTL recognize highly conserved HIV-1 epitopes.
Objective: To determine whether CTL can induce escape mutations in an immunodominant epitope of HIV-1.
Methods: HIV-1 IIIB was serially passaged in the presence of a CTL clone recognizing the MHC A2-restricted immunodominant epitope SLYNTVATL (Gag p17, aa 77-85). The virus was grown in either T1 (MHC A2+) or H9 (A2-). After each passage, the recognized epitope was sequenced, and the resultant virus was also tested for inhibition by the CTL clone. A variant that arose was tested for recognition by the CTL clone using synthetic peptide titration assays. Sequencing of this epitope in the patient from whom this CTL clone was derived was described in a previously published study from our group.
Results: After 2 passages, the virus in the MHC-matched T1 cells developed phenotypic resistance to inhibition by the CTL clone, and sequencing revealed a mutation to SLYNTIATL (9/16 sequences). Virus in T1 cells after the first passage or in control MHC-mismatched H9 cells remained fully wild-type by inhibition assay and sequencing. Peptide titration assays revealed that the variant peptide was approximately 1000-fold less recognized by the CTL clone. Examination of this viral epitope in the patient from whom this CTL clone was derived revealed that 1/4 sequences contained this mutation (SLFNTIATL) and 3/4 did not (SLFNTVATL).
Conclusions: HIV-1 epitope mutation can occur readily under selective pressure by CTL. Studies of the immunodominant epitope studied here have found that most HIV-1-infected individuals do not appear to develop resistance mutations in this epitope. The patient from whom this CTL clone was derived had a minority of sequences with this resistance mutation. These data suggest that the frequently observed lack of escape mutations in vivo may be due to ineffective CTL and/or privileged sites of viral replication.
Key Words: CTL, escape, HIV-1
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