Phenotypic expression of integrin membrane receptors on spontaneously proliferating CD8 cells in human T-lymphotropic virus type II (HTLV-II)-infected individuals

Spontaneous lymphocyte proliferation in the absence of exogenous stimulators was examined in asymptomatic HTLV-II-seropositive (n=12) and seronegative individuals (n=16). Mean spontaneous lymphocytic proliferation significantly increased on day 8 postculture in HTLV-II-infected individuals (5762±899 cpm) compared with normal controls (2034±925 cpm, P<0.01). The proliferating cells in infected individuals were predominantly T cells; neither B cells nor monocytes demonstrated any proliferation. Phenotypic analysis of cultured cells from individuals with HTLV-II infection demonstrated differential expression of integrin molecules as defined by anti-CD29 and anti-S6F1 (42.8±4.2 and 39.6±5.9%, respectively) on CD8 cells, as compared with day 0 peripheral blood mononuclear cells (PBMC) from infected individuals (19.7±3.5 and 19.9±1.9%, respectively) or normal controls (12.9±3.1 and 11.5±2.5%, respectively;P<0.001 for both comparisons). These CD8⁺ cells did not express CD16 or CD11b. The culture supernatants derived from the spontaneously proliferating cells had significantly increased levels of sCD8 and sCD25 (765±180 and 1805±320 U/ml, respectively) compared with those from normal controls (222±120 and 305±90 U/ml, respectively;P<0.01). Furthermore, culture supernatants derived from spontaneously proliferating PBMC from HTLV-II-infected individuals had no detectable levels of HTLV antigen and did not stimulate proliferation of PBMC from normal donors. These results suggest that the spontaneous proliferation in HTLV-II asymptomatic carriers is due to expansion of CD8 cells expressing integrin receptors which may serve as costimulatory molecules for their activation.