Neuroinflammatory changes in the brain, including infiltration of lymphocytes, particularly T cells, play a critical role in the pathogenesis of Parkinson's disease (PD).^{1, 2} Interestingly, in the peripheral blood of PD patients, a decrease in circulating lymphocyte counts occurs, mainly due to a decrease in T cells.^{1, 3} Furthermore, it has recently been reported that lower lymphocyte count might be causally related to the subsequent development of PD.⁴ Inspired by these observations, we aimed at assessing whether low lymphocyte count is associated with the subsequent development of the key milestones in PD's disease course, specifically cognitive impairment, with a particular attention to the apolipoprotein E (ApoE) ε4 allele, a crucial modifying factor in cognitive impairment.^{5, 6}

In this retrospective cohort study, using the Parkinson's Progression Markers Initiative data, 167 de novo PD patients were enrolled (Fig. S1) and were followed up for 2 years (Tables S1 and S2; Text S1). R scripts made for the analysis are freely available at http://dx.doi.org/10.17632/7s8sng9yn8.2 or https://github.com/KazutoTsukita/Mov_Disord_2021.

We primarily used the multivariate linear mixed-effects model adjusted for various covariates (age, sex, levodopa-equivalent dose, disease duration, and baseline severity of smell deficit and rapid-eye-movement sleep behavior). We observed that only in PD patients carrying ApoE ε4 allele, baseline lymphocyte count had significant interaction effect on the longitudinal decline in the Montreal Cognitive Assessment (MoCA) total score, such that lower baseline lymphocyte count was associated with accelerated MoCA score decline (carrier, the standardized fixed-effects coefficient of the interaction term (β_interaction) = 0.17 [95% confidence interval, CI: 0.04, 0.30], P = 0.01; noncarrier, β_interaction = −0.00 [95% CI: −0.10, 0.09], P = 0.94). When PD patients, with and without ApoE ε4 allele, were dichotomized using the median of baseline lymphocyte count (carrier, 1.72 × 10³/μL; noncarrier, 1.74 × 10³/μL) (Table S3), the interaction effect was apparent only in PD patients carrying ApoE ε4 allele (carrier, β_interaction = 0.45 [95% CI: 0.20, 0.71], P < 0.001; noncarrier, β_interaction = −0.03 [95% CI: −0.22, 0.15], P = 0.72) (Fig. 1A,B). The interaction effects of baseline lymphocyte count on the progression of specific domains of cognitive impairment did not reach statistical significance (Fig. 1C). Sensitivity analyses confirmed the robustness of our result in a range of follow-up periods (Table S4) and even when missing values were imputed (Table S5).

An interesting aspect of the present result is that baseline lymphocyte count was clearly associated with subsequent cognitive decline only in PD patients carrying ApoE ε4 allele. Given the importance of ApoE ε4 allele in blood–brain barrier (BBB) dysfunction and the role of circulating T cells in PD pathogenesis (Text S2),^{1, 7} our result might indicate the cooperative pathological role of BBB dysfunction and circulating lymphocytes in PD. Alternatively, the brain cortex of patients carrying ApoE ε4 allele may be particularly vulnerable to lymphocyte infiltration. Admittedly, this study has some limitations (Text S3); however, because many covariates were adjusted for, we believe that our result indicates that biological phenomenon reflected by the decrease in the lymphocyte count might actively exacerbate the pathology driving cognitive dysfunction in synergy with the APOE ε4 allele, thereby providing important clinical and pathophysiological implications.

大脑中的神经炎症变化，包括淋巴细胞，特别是 T 细胞的浸润，在帕金森病 (PD) 的发病机制中起关键作用。^{1, 2}有趣的是，在 PD 患者的外周血中，循环淋巴细胞计数会发生减少，这主要是由于 T 细胞减少。^{1, 3}此外，最近有报道称，淋巴细胞计数降低可能与 PD 的后续发展有因果关系。⁴受这些观察的启发，我们旨在评估低淋巴细胞计数是否与 PD 病程中关键里程碑的后续发展相关，特别是认知障碍，特别关注载脂蛋白 E (ApoE) ε4 等位基因，一个重要的修饰因子在认知障碍。^5、6

在这项回顾性队列研究中，使用帕金森病进展标志物倡议数据，招募了 167 名新发 PD 患者（图 S1）并随访 2 年（表 S1 和 S2；文本 S1）。用于分析的 R 脚本可在 http://dx.doi.org/10.17632/7s8sng9yn8.2 或 https://github.com/KazutoTsukita/Mov_Disord_2021 免费获得。

我们主要使用针对各种协变量（年龄、性别、左旋多巴等效剂量、疾病持续时间和嗅觉缺陷的基线严重程度和快速眼动睡眠行为）进行调整的多元线性混合效应模型。我们观察到，仅在携带 ApoE ε4 等位基因的 PD 患者中，基线淋巴细胞计数对蒙特利尔认知评估（MoCA）总分的纵向下降具有显着的交互作用，因此较低的基线淋巴细胞计数与 MoCA 评分加速下降相关（载体，交互项的标准化固定效应系数 (β_交互) = 0.17 [95% CI: 0.04, 0.30], P  = 0.01;非携带者, β_交互 = -0.00 [95% CI: -0.10, 0.09] , P = 0.94)。^{当使用基线淋巴细胞计数的中位数（携带者，1.72 × 10 3} /μL；非携带者，1.74 × 10 ³ /μL）对具有和不具有 ApoE ε4 等位基因的 PD 患者进行二分时（表 S3），相互作用效应仅是明显的在携带 ApoE ε4 等位基因的 PD 患者中（携带者，β_相互作用 = 0.45 [95% CI：0.20, 0.71]，P  < 0.001；非携带者，β_相互作用 = -0.03 [95% CI：-0.22, 0.15]，P = 0.72) (图 1A,B)。基线淋巴细胞计数对认知障碍特定领域进展的交互作用没有达到统计学意义（图1C）。敏感性分析证实了我们的结果在一系列随访期间的稳健性（表 S4），甚至在估算缺失值时（表 S5）。

本研究结果的一个有趣方面是基线淋巴细胞计数仅与携带 ApoE ε4 等位基因的 PD 患者的认知能力下降明显相关。鉴于 ApoE ε4 等位基因在血脑屏障 (BBB) 功能障碍中的重要性以及循环 T 细胞在 PD 发病机制中的作用（文本 S2），^{1, 7}我们的结果可能表明 BBB 功能障碍和循环淋巴细胞在 PD 中的协同病理作用。或者，携带 ApoE ε4 等位基因的患者的大脑皮层可能特别容易受到淋巴细胞浸润的影响。诚然，这项研究有一些局限性（文本 S3）；然而，由于调整了许多协变量，我们相信我们的结果表明，淋巴细胞计数减少所反映的生物学现象可能会积极加剧与 APOE ε4 等位基因协同作用的病理驱动认知功能障碍，从而提供重要的临床和病理生理学意义。