Contrasted TCRβ Diversity of CD8⁺ and CD8⁻ T Cells in Rainbow Trout

Teleost fish express highly diverse naive TCRβ (TRB) repertoires and mount strong public and private clonal responses upon infection with pathogens. Fish T cells express typical markers such as CD8, CD4-1 and CD4-2, CD3, CD28 and CTLA4. Fish CD8+ T cells have been shown to be responsible for antigen-specific cell-mediated cytotoxicity in in vitro systems using histo-compatible effector and target cells. We compare here the complexity of TRB repertoires between FACS sorted CD8+ and CD8− T cells from spleen and pronephros of rainbow trout. In contrast to human, while the TRB repertoire is highly diverse and polyclonal in CD8+ T cells of naïve fish, it appeared very different in CD8− lymphocytes with irregular CDR3 length distributions suggesting a dominance of activated clones already in naïve fish or the presence of non conventional T cells. After infection with a systemic virus, CD8+ T cells mount a typical response with significant skewing of CDR3 length profiles. The infection also induces significant modifications of the TRB repertoire expressed by the CD8− fraction, but for a different set of V/J combinations. In this fraction, the antiviral response results in an increase of the peak diversity of spectratypes. This unusual observation reflects the presence of a number of T cell expansions that rise the relative importance of minor peaks of the highly skewed distributions observed in unchallenged animals. These results suggest that the diversity of TRB expressed by CD8+ and CD8− αβ T cells may be subjected to different regulatory patterns in fish and in mammals.