Co-receptor choice by Vα14i NKT cells is driven by Th-POK expression rather than avoidance of CD8-mediated negative selection

Mouse natural killer T (NKT) cells with an invariant Vα14-Jα18 rearrangement (Vα14 invariant [Vα14i] NKT cells) are either CD4 ⁺CD8^- or CD4^-CD8^-. Because transgenic mice with forced CD8 expression in all T cells exhibited a profound NKT cell deficit, the absence of CD8 has been attributed to negative selection. We now present evidence that CD8 does not serve as a coreceptor for CD1d recognition and that the defect in development in CD8 transgene homozygous mice is the result of a reduction in secondary T cell receptor α rearrangements. Thymocytes from mice hemizygous for the CD8 transgene have a less severe rearrangement defect and have functional CD8⁺ Vα14i NKT cells. Furthermore, we demonstrate that the transcription factor Th, Poxviruses and Zinc finger, and Krüppel family (Th-POK) is expressed by Vα14i NKT cells throughout their differentiation and is necessary both to silence CD8 expression and for the functional maturity of Vα14i NKT cells. We therefore suggest that Th-POK expression is required for the normal development of Vα14i NKT cells and that the absence of CD8 expression by these cells is a by-product of such expression, as opposed to the result of negative selection of CD8-expressing Vα14i NKT cells.