Defining Successful Immune Reconstitution (IR) after Allogeneic Hematopoietic Cell Transplantation (HCT)

Allo-HCT is curative for hematological malignancies that are refractory to conventional chemotherapy. However, GVHD, CMV reactivation and relapse are common complications that either impair immune recovery (IR) or are secondary to the profound immune deficiency associated with these complications. We reasoned that patients who lacked these and survived beyond 1 year ‘behaved' as if they had reconstituted functionally. To address this hypothesis we analyzed T, B and NK IR in 423 allo-HCT patients with hematologic malignancy without TRM in the first year.  Patients who lacked GVHD, CMV reactivation and relapse were assigned to the “successful” IR group (n=115), while those who had any of these were assigned to the “suboptimal” IR group (n=308). Because some “successful” patients were CMV seronegative (CMV-) and thus at low risk for CMV viremia, we further divided this group into “successful-CMV+” (n=50) and “successful-CMV-” (n=65). In terms of 5-year DFS, “successful-CMV+” patients had the best outcome followed by “successful-CMV-” and “suboptimal” patients (90% [95% CI: 76-96%] vs 57% [36-72%] vs 43% [37-49%], p<0.01, see Figure 1 for pairwise comparisons).  Quantitative analysis of lymphocyte subsets were evaluated at days +30, +60, +100, +180 and +365 post-transplant. Interestingly a greater proportion of “successful CMV+” patients had CD3+ T cell counts within the normal range (>1480/ul) by D+60 as compared to that in “successful-CMV-“ and “suboptimal” patients (57% vs. 13% vs. 24%, p=0.0005 and p=0.0025, respectively).  This was primarily driven by faster CD3+CD8+ recovery to normal levels (64% vs. 15% vs. 26%, p=0.0004 and p=0.0016, respectively). These findings were durable over one year, with the “successful-CMV+” patients having significantly higher absolute numbers of CD3+CD8+ T cells at nearly every time point compared to the other two groups.  Similarly, a significantly higher proportion of patients reached the normal range for B cells in the “successful-CMV+” and “successful-CMV-” groups relative to “suboptimal” IR group. After antigen encounter, naïve T cells differentiate into effector memory (EM) and central memory (CM) cells with some of the EM cells reacquiring CD45RA (EMRA); therefore, we evaluated the three groups for recovery of these T cell subpopulations.  “Successful-CMV+” patients had significantly higher numbers of CD4+ and CD8+ EM and EMRA cells at nearly all time points compared to the other groups. Using a multivariate mixed model and taking time since transplant, donor stem cell source and recipient age into consideration, patients in “successful-CMV+” IR group showed higher ALC as well as higher absolute numbers of CD3, CD4EM, CD4EMRA, CD8EM, CD8EMRA, and NKG2C+CD57+ relative to the other group , Table 1). Together, these data define the phenotypic profile of the successful IR with the unexpected findings of better survival in CMV+ seropositive patients who are able to control CMV reactivation (and without GVHD or relapse). These results highlight the intriguing relationship between CMV and the human immune system and further suggest that strategies to control CMV reactivation (drugs, cellular therapy or vaccination) may have an unexpected impact on transplant outcomes.

Figure 1:  5 Year DFS and intergroup comparisons.

Table 1: Multivariable Regression Analysis on Lymphocyte Recovery Between Patient Groups.  Linear mixed model was used on log transformed absolute counts at multiple time points with covariates of group, time since transplant, stem cell source, and recipient age. Regression coefficients reflect the “average” effect over time adjusted for stem cell source and recipient age.

Lymphocyte Subsets	Successful CMV+ vs  Successful CMV-		Non- Successful vs    Successful CMV +		Suboptimal vs    Successful  CMV-
	Coefficient	P value	Coefficient	P value	Coefficient	P value
ALC	0.29	0.014	-0.32	0.001	-0.04	0.664
CD19	0.09	0.77	-0.96	0.0001	-0.88	<0.0001
CD3	0.47	0.004	-0.38	0.006	0.09	0.454
CD3CD4	0.17	0.269	-0.33	0.014	-0.16	0.169
CD4 Naive	-0.01	0.96	-0.25	0.22	-0.26	0.131
CD4CM	0.09	0.59	-0.25	0.099	-0.15	0.228
CD4EMRA	0.84	<0.0001	-0.38	0.034	0.46	0.003
CD4EM	0.62	0.0004	-0.50	0.001	0.12	0.35
CD3CD8	0.95	<0.0001	-0.49	0.005	0.46	0.002
CD8 Naive	0.42	0.025	-0.30	0.056	0.11	0.403
CD8CM	0.59	0.006	-0.31	0.085	0.27	0.08
CD8EMRA	1.53	<0.0001	-0.50	0.028	1.02	<0.0001
CD8EM	1.38	<0.0001	-0.53	0.03	0.85	<0.0001
NKG2C+CD57	0. 93	<0.0001	-0.39	0.05	0.54	0.0005