Background: Alemtuzumab (Campath-1H) is a humanized mAb directed against CD52 that depletes circulating but not bone marrow lymphocytes. A transient increase in proportion of circulating Treg is observed following its administration in humans. Very little is known about its influence on lymph nodes (LN), which play a central role in antigen presentation and serve as the main reservoir of circulating lymphocytes and Treg.

Methods: We investigated the influence of alemtuzumab on lymphocyte subsets and their subsequent repopulation in LN of Indonesian cynomolgus monkeys. Mesenteric and inguinal LN were harvested from (i) healthy controls (Gp 1, n=10), (ii) monkeys given various doses of alemtuzumab, but no transplant (Gp 2, n=6), and (iii) monkeys given 40 mg/kg alemtuzumab and an ABO-compatible heart transplant (Gp 3, n=5). LN were harvested at various time-points up to 1 yr after alemtuzumab. Absolute numbers of lymphocyte subsets, effector and memory T cells, and Treg were determined by flow cytometry.

Results: Normal LN contained 770 ± 290 x10³ cells/mg tissue. Significant depletion of 75% and 87% to 191 ± 32 and 100 ± 81 x10³ cells/mg was observed in Gps 3 and 2 at 2 d and 2 wks, respectively, which recovered to 538 ± 138 x10³ cells/mg 12 mths post-alemtuzumab (Gp 2). CD3⁺T and CD20⁺B cells were equally depleted at d 2, but while CD3⁺T cells remained significantly reduced at 12 mths (42% total lymphocytes vs 71% in controls) CD20⁺B cells doubled (51% vs 25% in controls). Within CD3⁺T cells, the incidence of CD4⁺ T effector memory cells increased to 68.6% in Gp 3, and 70.0% in Gp 2, at 3 and 12 mths post-alemtuzumab, respectively, compared to 45.4% in controls, while decreased proportions of CD4⁺ naÏve T cells were observed. CD8⁺ T memory cells showed a similar trend. CD4⁺FoxP3⁺ Treg were reduced from 11.8% to 4.9% of CD4⁺ T cells by 3 mths.

Conclusions: Alemtuzumab profoundly depletes lymphocytes in monkey LN. CD3⁺ T cells are the the major population affected. Within recovering CD3⁺T cells there is an increased proportion of effector memory T cells. No difference between alemtuzumab only (Gp 2), and alemtuzumab plus transplant (Gp 3) was observed. Unlike observations on circulating lymphocytes, the incidence of Treg in the LN was reduced by alemtuzumab.

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