Our results are in contrast with the initial in vitro data showing that anti-CD3 or PMA-activated human T cells were further induced to proliferate by CD69 targeting, but are in agreement with a posterior observation indicating that Ag-specific T cell proliferation was unaffected in CD692/2 mouse T cells in vivo [27]. Of notice, not all the anti-CD69 MAbs tested in initial works were reported to have this proliferation-enhancing effect on human T cells [43–44]. Altogether, more physiological in vitro and in vivo data argue against the initially proposed role for CD69 as a costimulatory molecule. When studying the influence of CD69 in a physiological setting in which the priming occurs to endogenous na¨ıve T cell pools of various frequencies and TCR affinities, such as the Vaccinia virus infection, we observed that CD69 targeting or deficiency did not alter the size of the VACV-specific T cell population at the peak of the primary response. This is in support of our previous hypothesis that, if we found a slightly smaller population of Lm-specific T cells in Lm-infected CD692/2 mice [10], it was not due to an intrinsic defect in Ag-specific T cell priming. Instead, it could be a collateral effect of the Lm-induced, type I IFN-mediated massive lympho- cyte apoptosis [45] affecting also primed Lm-specific T cells. Of notice, the defect in the control of Lm infection in CD692/2 mice was noticeable as soon as day 1 post-infection (and thus, induced by differences in the innate immune response), was associated to increased type I IFN levels and increased spleen cell death, and was not observed in lymphocyte deficient mice. In contrast to Lm infection, Vaccinia virus infection does not lead to massive lymphocyte apoptosis, and this might be the reason for it not inducing this innate immunity-based difference in the size of specific T cell populations. References 1. 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Laffon A, Garcia-Vicuna R, Humbria A, Postigo AA, Corbi AL, et al. (1991) Upregulated expression and function of VLA-4 fibronectin receptors on human activated T cells in rheumatoid arthritis. J Clin Invest 88: 546–552. 14. Ortiz AM, Laffon A, Gonzalez-Alvaro I (2002) CD69 expression on lymphocytes and interleukin-15 levels in synovial fluids from different inflammatory arthropathies. Rheumatol Int 21: 182–188. 15. Portales-Perez D, Gonzalez-Amaro R, Abud-Mendoza C, Sanchez-Armass S (1997) Abnormalities in CD69 expression, cytosolic pH and Ca2+ during activation of lymphocytes from patients with systemic lupus erythematosus. Lupus 6: 48–56. CD69 Role on T Cell Priming Altogether, the in vivo and in vitro data point to that targeting of CD69 expressed or up-regulated on DC does not affect their Ag processing, Ag presentation or costimulation capacity, and that CD69 does not function as a costimulatory molecule on DC or on T cells. These results do not rule out, though, that CD69 could affect other important aspects influencing or being determined by DC-T cell interaction, such as DC polarization or T cell polarization and programming. In this regard, CD69 has recently been reported to inhibit Th17 differentiation in Ag primed CD4+ T cells [46]. Taking this into account, it could be hypothesized that the exacerbated forms of Ag-specific T cell-dependent diseases reported in CD692/2 and anti-CD69 2.2-treated mice are not due to differential priming of Ag-specific T cells but might rather be owed to skewed polarization of primed T cells. On the whole, this work contributes to resolving a previous controversy and to shift the focus on CD69 towards its effect on T cell polarization, as it is starting to be apparent, rather than on the extent of T cell priming and T cell costimulation. Acknowledgments We are grateful to Ms. Isabel Pahissa-Garcia for excellent proofreading of English. 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