To the editor
Secondary lymphoid organs are widely understood to play a pivotal role in coordinating adaptive immune responses, though direct demonstrations are few. Most available evidence use as their model the alymphoplastic (aly/aly) mouse which lacks peripheral lymph nodes as well as Peyer's patches1,2,3. Most recently, Lakkis and colleagues demonstrated that splenectomized aly/aly mice are rendered immunologically 'ignorant' of their subsequent cardiac allograft, and aly/aly mice with spleens in place permanently accept skin allografts. The authors conclude that secondary lymphoid organs are essential for mediating rejection. Although they are undoubtedly necessary in certain responses, it is unclear to us that secondary lymphoid organs play an essential role here. Apart from the absence of lymph nodes, aly/aly mice host a variety of other serious immune defects which might confound the authors' findings, including depressed baseline immunoglobulin production and isotype switching, defective T-cell function and faulty homing responses1,5.
Lymphotoxin-α–deficient (Lta−/−) and lymphotoxin-β-receptor–deficient (Ltbr−/−) mice are similarly devoid of lymph nodes and Peyer's patches, but share few other defects additionally associated with aly/aly mice6,7. Their T-cell responses indeed appear unperturbed8,9. Lta−/− mice reject allogeneic skin grafts in near identical tempo to wild-type controls (Fig. 1a), a finding confirmed in Ltbr−/− and splenectomized Ltbr−/− recipients (MST:14.2d, n = 9). Moreover, splenectomized Lta−/− and Ltbr−/− recipients remain capable of rejecting BALB/c cardiac allografts, sometimes in as few as 21 days (Fig. 1b). The delayed rejection points to the importance but also the dispensability of secondary lymphoid organs for rejection of cardiac allografts. The finding that Lta−/− and Ltbr−/− recipients reject heart transplants in the absence of secondary lymphoid organs challenges the wider significance of conclusions Lakkis and colleagues draw from their data, and suggests that concomitant intrinsic defects found uniquely in aly/aly mice—apart from lymph-node agenesis—may have contributed significantly to their findings. The reciprocal adoptive transfer controls they performed to address these concerns are complicated by issues of homeostatic expansion and survival.
a, Lymph nodes are not essential for the rejection of skin allografts. Full-thickness tail skin was transplanted from 5–8-wk-old BALB/c mice to the dorsal flank area of 5–8-wk-old C57BL/6 or B6-Lta−/− recipients. Rejection was defined as graft necrosis more than 80%. Time course of rejection of allogeneic (BALB/c) skin grafts is near identical in Lta−/− (░; n = 5) and wild-type (♦; n = 5) mice. Similar results were also obtained when donor abdominal skin was used. b, Lymph nodes and spleen are not essential for the rejection of cardiac allografts. Cardiac allograft transplant was performed using BALB/c donors and C57BL/6 (♦; n = 7), splenectomized B6-Lta−/− (░; n = 5) or splenectomized B6Ltbr−/− (▴; n = 4) recipients. Allograft survival was assessed by palpation. Rejection was defined as cessation of heart beat. Splenectomized B6-Lta−/− and B6-Ltbr−/− recipients show delayed, but fully competent rejection.
The findings of Lakkis and colleagues in aly/aly mice are no doubt important. However, given the pleiotropic effects of the aly mutation and in light of conflicting findings in Lta−/− and Ltbr−/− mice when compared with aly/aly mice, a more cautious interpretation of the data might be prudent. It is likely that the aggregate of diverse immune defects in the aly/aly phenotype together, rather than lymph node and Peyer's patch agenesis alone, may be responsible for the immunological 'ignorance' witnessed in aly/aly mice. The salient consideration is that other studies of immune responses studied in aly/aly mice might overestimate the involvement of secondary lymphoid organs1,2,3.
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Chin, R., Zhou, P., Alegre, ML. et al. Confounding factors complicate conclusions in aly model. Nat Med 7, 1165 (2001). https://doi.org/10.1038/nm1101-1165a
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DOI: https://doi.org/10.1038/nm1101-1165a
