Delayed infection after immunization with a peptide from the transmembrane glycoprotein of the feline immunodeficiency virus

Abstract

Recent advances in the quantitative assessment of viral burden, by permitting the extension of criteria applied to assess the efficacy of vaccines from all-or-none protection to diminution of the viral burden, may allow the identification of original immunogens of value in combined vaccines. Peptides corresponding to three domains of the envelope glycoproteins of feline immunodeficiency virus that are recognized during natural infection were used to immunize cats. After challenge with a primary isolate of feline immunodeficiency virus, the development of acute infection was monitored by quantitative assessment of the viral burden in plasma and tissues by competitive reverse transcription-PCR, by measurement of the humoral response developed to viral components, and by lymphocyte subset analysis. Whereas immunization with two peptides derived from the surface glycoprotein had no effect on the early course of infection, immunization with a peptide derived from the transmembrane glycoprotein delayed infection, as reflected by a diminished viral burden in the early phase of primary infection and delayed seroconversion. This peptide, located in the membrane-proximal region of the extracellular domain, has homology to an epitope of human immunodeficiency virus type 1 recognized by a broadly neutralizing monoclonal antibody. These results suggest that lentivirus transmembrane glycoproteins share a determinant in the juxtamembrane ectodomain which could be of importance in the design of vaccines against AIDS.

FIG. 1

Diagrammatic representation of the envelope of FIV. Sites of proteolytic cleavage eliminating an amino-terminal signal peptide (arrowhead) and generating mature surface (SU) and transmembrane (TM) envelope glycoproteins (arrow) are indicated. Peptides SU2, SU5, and TM3 were used as immunogens. Positions correspond to the amino acid sequence of the envelope glycoprotein of the Wo strain. Symbols: ▪, peptides; , signal sequence; , putative membrane-spanning domain.

FIG. 2

Lymphocyte subset analysis. The number of circulating CD4⁺ lymphocytes during immunization and, following challenge (day 0), during acute infection are shown. Data are given as means. Error bars represent standard deviations.

FIG. 3

Viral burden in plasma during primary infection. The viral burden, as determined by competitive RT-PCR, is expressed as the number of copies of viral RNA per milliliter plasma. The dotted line, denoting plasma samples for which viral RNA was not detected, is placed at 2,860 copies/ml, which corresponds to the threshold of detection when plasma was diluted 1/2 (see Materials and Methods). Since plasma samples collected at 3 weeks had to be diluted 1/10 as a result of an inhibitory substance, the viral burden in the three 3-week samples for which no viral RNA was detected must be considered to be less than 14,300 copies/ml of plasma. Plasma samples for which viral RNA, although detected, was present at less than 30 copies per reaction are denoted by a dashed line, corresponding to 8,580 copies/ml of plasma.

FIG. 4

Viral burden in lymphoid tissues. The viral burden in lymphoid tissues 5 weeks after challenge, as determined by competitive RT-PCR, is expressed as the number of copies of viral RNA per microgram of total RNA. Symbols: ▪, Axillary ganglion; , spleen; □, thymus.

FIG. 5

Diagrammatic representation of the ectodomain and membrane-spanning domain of the transmembrane glycoproteins of HIV-1 (LAI) and FIV (Wo) and alignment of the FIV TM3 peptide with an HIV-1 peptide comprising the 2F5 B-cell epitope. The TM3 epitope (QQLGEWED) best recognized during FIV infection and the 2F5 epitope (ELDKWA) are indicated. Symbols: ░⃞, fusion peptide; ▧, leucine zipper region; ▨, membrane-proximal α-helix; , membrane-spanning domain.