In our article published in The Journal of Immunology in June 1, 2005 (1), we reported that lymph nodes from nackt mice (CTSLnkt/CTSLnkt) are hypertrophied, showing a normal absolute number of CD4+ T cells and a marked increase in the number of CD8+ T lymphocytes. Correlatively, extracellular matrix (ECM) components were found to be increased. Contrarily, in nackt thymus, laminin, fibronectin, and collagen I and IV are markedly decreased, with an augmented output of CD8+ cells. We also reported that a mutated form of cathepsin L can be detected in different organs in nackt mice. These results demonstrate that the nackt mutation in the Ctsl gene influences the levels of ECM components in lymphoid organs, the thymic output, and the number of T cells in the periphery, thus broadly affecting the immune system.

In a letter to the editor, Benavides et al. (2) show that it is possible to detect a mutated cathepsin-L (CTSL) in nackt mice, indeed confirming our results. As stated in our article, it is clear that the nkt mutation produces a mutated CTSL devoid of its classical proteolytic activity. The 118-bp deletion of nkt mice (2) involves the end of exon 6 and almost all of exon 7, sequences partially coding for the protein H and L chains. As a consequence of this deletion, a stop codon would appear leading to a truncated peptide lacking the last 60 aa, which are included in the active site of this protease (3). However, they claim that the nackt allele clearly behaves as a recessive loss-of-function mutation where heterozygous mice are phenotypically indistinguishable from wild-type mice and homozygous mutants exhibit the same phenotype as Ctsl knockout (KO) mice. Regarding the differences between wild-type and +/nkt mice, Benavides et al. (2) have reported that the skin of +/nkt mice shows no major difference with that of +/+ mice. However, in our hands, significant differences in the level of expression of integrins between +/+ and +/nkt mice can be detected (Fig. 1; I. Piazzon and I. Nepomnaschy, manuscript in preparation). As an example, data on Fig. 1 show that the percentage of lymph node CD4+ cells expressing high levels of α6 integrin chain is 2-fold increased in +/nkt vs +/+ mice. Thus, on one hand, it cannot be discarded that haploinsufficiency affecting different traits can be detected in +/nkt mice, as it has been described for the skin and the trabecular bone volume of CTSL KO mice (B6;129-Ctsltm1Alpk) (4). On the other hand, we have suggested the possibility that the presence of a mutated CTSL devoid of proteolytic activity in nackt mice could lead to the appearance of phenotypic differences between CTSL KO and nackt mice. It is unclear whether the nackt phenotypes we have described (1) are shared by the CTSL KO mice. The total cellularity of spleen and thymus in wild-type and CTSltm1Cptr/CTSltm1Cptr mice has been reported to be comparable (5), and nackt mice also show normal number of thymocytes and splenocytes. It has not been reported that lymph node cellularity is altered in CTSL KO mice, while nackt mice show increases in the number of lymph node cells. Nakagawa et al. (5) had shown that the percentage of CD8+ thymocytes is increased in CTSltm1Cptr/CTSltm1Cptr mice, while no differences could be detected in nackt mice. To our knowledge, the additional phenotypes, including differences in extracellular matrix components in the thymus and lymph nodes that we have described, have not been investigated so far for CTSL KO mice. Chauhan et al. (6) had suggested that human CTSL lacking the 16 C-terminal amino acids is retained in the endoplasmic reticulum presumably because of its improper folding. The possibility exists that the presence of a mutated CTSL in nackt mice could lead to the appearance of phenotypic differences between KO and nackt mice. Direct testing of this hypothesis would be required.

FIGURE 1.

α6 integrin chain expression in CD4+ lymph node cells of +/+, +/nkt, and nkt/nkt mice. Lymph node cells from +/+, +/nkt, and nkt/nkt littermates were stained with anti-CD4 and anti-CD49e and analyzed by flow cytometry. Values represent the mean percentages of cells expressing high levels of α6 integrin chain ± SD (n = 6) within CD4+ cells. ∗, Significantly different from +/+ (p < 0.01); from nkt/nkt (p < 0.01).

FIGURE 1.

α6 integrin chain expression in CD4+ lymph node cells of +/+, +/nkt, and nkt/nkt mice. Lymph node cells from +/+, +/nkt, and nkt/nkt littermates were stained with anti-CD4 and anti-CD49e and analyzed by flow cytometry. Values represent the mean percentages of cells expressing high levels of α6 integrin chain ± SD (n = 6) within CD4+ cells. ∗, Significantly different from +/+ (p < 0.01); from nkt/nkt (p < 0.01).

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